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create my own push notification service without fcm , pusher ,

I want to create a notification system provider that is not based on Google services or similar. In fact, I want to get information about its overall architecture and needed Android sdk functionality.
The most weird point for me is to understand how to send a notification to an Android device.
I mean, how can I identify the Android device on which my application is installed from millions and millions of other Android devices on the Internet?
And how do I send information to him?
Should I use sockets for this or similar stuff?

simple answer: YOU CAN NOT
before everything else i should correct your question, pusher and FCM are not in the same group at all! you can build somthing like pusher or oneSignal or etc but you can not build something like FCM/APNS
you should understand three simple yet important sentences below:
when you want to pull anything from place_1(e.g. api) to place_2(e.g. browser_client) you most have an identifier of the place_1_resource (which commonly is the uniform-resource-locator of api)
when you want to push anything from place_1(e.g. notification_central_server) to place_2(cellphone_client) you most have an identifier of the place_2_resource
you must know the differences between a real server push with server-push-like technologies like long-pulling or ... and you should be aware that what is intended in this concept is a real server push not any kind of pulling with a push jacket!
if you don't have any identifier for a cellphone which you want to send it a notification, your server dont know where to send that notification so we need a resource_identifier_like for cellphones which is actually a device_token_like and you have just one approach to get this device_token_like and that is the FCM/APNS
FCM is like a dns server containing all identifiers of every android device that google supports (almost every android device) and APNS is just the same but for apple devices
note1: even if your app can obtain it's corresponding device device_token_like it can not be used for push notification if its not registered on FCM/APNS
so when you get that device_token_like identifier of your desired clinet_device now you can use different approaches for sending sth to that clinet_device. there are several approaches like SSE, Webpush, HTTP_server_push, Pushlet and etc but none of these approaches supported by mother_companies of these devices, the only approach that is completely supported and standard is the same approach that FCM/APNS official websites suggests
for example an iranian Incorporation named najva uses webpush to send notifications because of USA sanctions but webPush method works good on browsers and android devices but they didn't even apear on an apple devices
finally i should say that i admire your curiosity to less using anything from a benefit_based Inc. like FCM/APNS in your developing but i strongly recommend these articles and books for you cause i think you didn't learn enough:
wikipedia of push technology
story of some guy who tries to make his own push notification service
Push Technology A Complete Guide - 2020 Edition
Data Push Apps with HTML5 SSE

Short, direct answer
You can't (At least till you create your own ROM)
TL;DR, Reason why?
Before you build your own push notification server, you first need to know how it works internally in android.
Whenever you/your server sends a push notification message to the android client, the SDK processes it and shows you the notification. But when your app is not running (or being killed), your app cannot respond to it since it was not running. In such a case, your notification message is sent to a system service which is known as Google play service. For this even to work, you will first need to bind your app with Google play service and that is what FCM does. FCM SDK registers your app to the operating system service on the first initialization. That FCM service is opened to a port which listened to the incoming message from the server and when it receives the message, it publishes a notification on behalf of your app with a PendingIntent containing the data. Then the PendingIntent is delivered to your app when the user clicks it and then finally your app process the data (or the push message)
So basically, for your server to communicate with the client, It first needs to communicate with the FCM service and for that, FCM gives you a token which identifies the application to register with the internal Google play service.
Simplified furthermore, the workflow is as follows:-
Server send push message ---> FCM ---> Google play service,
If your app is running, it is directly handled by the client SDK So, Google play service --> Your app
If not, then it is delivered by the service itself using PendingIntent So, Google play service --> PendingIntent --->| Publish notification
Totally impossible, Workaround?
There is nothing like impossible because an absolute impossibility doesn't exist. Saying impossible generally means near to impossible. (This is similar to math where also we say tends to infinity because no one has achieved it yet).
To make it work, you need to somehow bind your app to the Google play service and you can't because Google hasn't exposed any direct API to do that. The only possible way is using the FCM ;-) (Bad luck again). So the only possible way is to build your own custom ROM with a custom push service that acts as a client for your Push server and a Server for your Push client (which is your app).
Since the above option tends to impossibility, you have to choose a workaround.
The best among the worst workarounds are:-
To make a malicious SDK.
Malicious because it needs to keep the app running in the background with a service that is connected to a WebSocket endpoint of your server. (Harder in new android versions).
Make use of a database where your push notification is saved and your app checks it periodically using AlarmManager.
Hope you have got the point.

Push Notifications for different platforms and devices in China

I read a lot of articles about how to implement Push Notifications with Cordova (and Ionic 2) and I'm not sure how to implement that in China when not using Google's push server.
I think using https://github.com/phonegap/phonegap-plugin-push is a good idea. But all the Howtos are focussing on creating an account in Google's FCM which is not really helpful in China. I doubt Chinese phones use Google to poll for notifications. Which leads to my next question.
I was assuming that there exists only one central push service for all platforms (Android, iOS and Windows) and vendors for the following reasons:
One platform is only polling from one fixed push server to save battery life.
If an app (server side) is sending a push notification, it does not know anything about the receiver's device, so it can only count on a centralized service delivering the message to the device regardless of platform or vendor.
But do Apple, Google and Microsoft all use the same push service? I doubt it, and that's what makes me confused. Also, if I'm in China, as Google's services are unstable here, the Android phones distributed in China are not using Google and even not install Google Play Store.
When I install a Chinese app (hosted in China) on a German phone (Android) I still get all notifications for that app even if I'm obviously polling from Google's push server. That means, the notification messages of the Chinese app is somehow delivered to Google.
The only explanation for that is that the different push services are working together. Is that the case? If so, than does it make a difference where I create an account (Google FCM: https://medium.com/#ankushaggarwal/gcm-setup-for-android-push-notifications-656cfdd8adbd#.rr2lfivfh), it is still usable on different push servers? Also, when I send a notification, could I use any server all over the world?
How can I find out which push server my phone is connected with (Android)?
A lot of confusing here, I really appreciate if someone could bring light to that topic :-)

The reason GCM / FCM does not work in China is simply because the Great Firewall of China blocks connectivity to all the various Google services, FCM included.
Also, most, if not all, China-sold phones do not come with Google Play Services, a requirement for FCM to work.
Even if you were to take a German phone, as you mentioned, into China, it would not receive notifications albeit having Google Play Services, due to the firewall.
But there are alternative push notification gateways you could use instead, which are far more reliable and do not depend on Google Play Services. One of those is Pushy.
Pushy (https://pushy.me/) offers a highly-reliable push notification gateway for Android apps that works independently from FCM, using its own background MQTT connection. It also offers iOS support so you can push all of your users via a single API for cross-platform support.
Full disclosure: I founded Pushy.

With Cordova, you can use the plugin cordova-plugin-xiaomi-push done with the Xiaomi Android SDK. I have tested and it works well with all the phones when the application is running. And even when the app is closed with Xiaomi phones :)
The advantage to use Xiaomi is that their developer website is in English and their server's APIs are simple.
There are also 2 plugins for Huawei and Oppo, which work well with the respective brand phones, even if the app is closed. But the API side is more complicated than with Xiaomi. To cover all the supplier with notifications when app is closed, it seems necessary to build one version per manufacturer/stores which include the required plugin (That is possible using a bash script) + build a push server that would send the notification to the right provider.

Google FCM notification not works in china country. You have to use some different push notification provider like Pushy.
For China Country, I used pushy service. You can go through this link if you wanna interested to use pushy service. Please check this link.This will help you

NodeJS to android app communication

I'm making a NodeJS server and an Android app that goes with it. I need to send alerts to my mobile app users that's coming from the web server. Something like a a chat app on the server and the android users getting the messages also (Facebook messenger?).
I have the server ready and Android app communicating to its database (for user login/getting user info/etc) using REST. I just need to figure out how to send info to the Android app WITH THE SERVER INITIATING COMMUNICATION. Any advice on where to start?

Start by looking at Google Cloud Messaging.
It's a free service that accepts messages from your server-based program and queues it to be sent to phones with your app when the phones are online.
Here is the overview on Android Developers.

YES!
Working with NodeJS, you have probably already heard of Socket.IO.
I'd say: Start there!
For your app to communicate with the server, there's also the java lib. These are great tools to start making any good real time service.
After you get along with this, you should also check GCM as #krisLarson suggested. And talkign about GCM, you have Node-GCM to make your work even easier.
Good luck!

Android Messages - Development vs Distribution

Forgive me for not knowing the correct Android termonology. I come from an iOS background so I'm trying to learn how to do two things correctly in the Android world.
Control "Badges" for Android
Send "Push Notifications" via Distribution (production) vs Development.
Fortunately I found the answer to #1 which is to use Status Bar Notifications thanks to this question.
Now for #2. I already have a GCM setup and can send notifications out to all registered devices. However, sometimes I want to be able to send notifications to all user's phones, and sometimes only to "development" devices. With iOS this is distinguished by devices that have been downloaded from the App Store / Ad-Hoc environment (Distribution) and devices that have been physically plugged into a computer which complies the source onto their device (Development). Is there anything similar to this for Android?

There is no difference between development and distribution in Google Cloud Messaging. All the messages are sent from your server to the same GCM endpoint.
If you want to distinguish between development devices and non-development devices, you'll have to manage it on your server's DB (for each registration ID you store in your server, add a flag that says if it's a development device or not).
EDIT :
Actually there's something else you can do, though I'm not sure it's such a good idea. When the app registers to GCM, it supplies a sender ID (which is a Google API project ID). You can use two different project IDs for development release and production release. The registration IDs returned by the registration process are tied to the project ID. Now, when you send a GCM message from your server, you send it using an API key that is tied to a project ID. If you use the API key that is tied to your development project ID, only registration IDs which are tied to that project ID (i.e. they originated from devices that have the development build) will work. Sending messages with "production" registration IDs will result in MismatchSenderId error (which is similar to APNS in the way that sandbox device tokens are invalid in the production environment and vice versa). I'm not sure that's a good idea, because you don't want to rely on errors from Google for your logic. If you want to send messages only to a subset of your clients, you should manage this subset in your DB.

How to send data from Server to Android Directly without invoking first from the android

How can i send a data from Server to android directly without waiting for the request received from the Android.
I have found one solution that is C2DM but it require separate Google account to use it Now i am afraid if i install the same app on two different android will it work or not
Plz suggest if u have some other mechanism.
Note: I dont want to use Network socket.

It will work. You need a gmail account registered for each android device AND a gmail account registered for sending out C2DM.
Alternatively you should make your own C2DM by polling your server at a regular interval (not recommended though).