Develop Reference

Apps not in google play store but play protect recognizes their developer

When the play protect says "it do not recognize the developer" when installing the application it means it has not seen the signature used in the application before in any play store application right?
But I have a question, we all have seen certain apps that are not in the play store but we use them now and then.
For example, we have the Fortnite mobile app, I looked into the web about the reason why it's not in the play store and there I found that they distribute the apk from their website directly because they do not want to pay google play the 30% of the in game purchase ammount that they charge from every application for advertising the application and distribution of the app.
There is also the F-Droid application that lets you download other open-source applications which are not available on the play store.
And many others like AdAway, Lawnchair, Venced Manager etc.
There are also modded applications of Prime Videos, Netflix, Spotify and completely separate apps like HotFlix, TeaTv etc. similar to them.
When we install these apps from outside the play store we don't get any error saying "Play protect doesn't recognise this app's developer, apps from unknown developers may be harmful".
So my question is do they use any signature that is for general use which is also known to google play protect like those open-source licenses we see in github while creating a new repo? or do they all have uploaded their apps in the play store and got banned, denied their policies, or something else?
I am asking this question because I could not find a satisfactory answer anywhere and nobody writes about it anywhere, if there are any mod creators reading this then they can help me out too 😉.
Note: Turning the play protect off is not a solution and I don't do that while installing other apps mentioned above so don't reply with that 😊.

When you release a signed .aab file to Google Play then it will be from a signed Google Partner, (I think that is what it is called). If you download an app that has not been cataloged as an Approved Signature with Google Play, then it is considered side loading.
Side loading is basically installing and .aab or .apk file that has not been verified by Google. So when you side load (install app away from Play Store) Google doesn't recognize the signature. So therefore it says this app is from an untrusted source.
So basically it boils down to whether or not Google has reviewed and approved the signature. This just a security measure that Google puts in place so that it categorized the signature as trusted or not trusted.
It's the same with desktop applications. If it doesn't have a certificate then it's from an unknown publisher.
So with Fortnite the reason why it's not on the Google Play store is because of Google Policy violations with Epic Gaming. So since they would not correct the violation of policy they were removed. So now Fortnite has an .apk file that will run on Android but it is no longer an approved signature in the Google Play database.
That's why Fortnite says it's an untrusted or unrecognizable signature. So it boils down to whether a signature has been added to the database.
You can go into developer mode on your phone and turn on the side loading option and you won't get that message.

Accessing Google Play Games out of Google Play Store

I have developed a game using Unity and I have used Google Play Games in it. It works good for testers and also for public users when I publish it on Google Play Games. The problem is I want to publish it on a local app store (and not Google Play Games) and it does not work this way! Is there any trick about it?

Yes, this is possible. It does even work if you sell your app through google play and on another platform. But the device needs to have the google play games app installed, so it does not work on Amazon devices.
Let’s assume your app is called “TheGame”.
Go to the Google Play Developer Console to “All application”:
Add another app name call it “TheGame (DRM-free)”
Create an empty APK that signed with a keystore and a different bundle identifier (if you already sell it on Google Play)
Upload the APK to alpha or beta testing, but not production.
Complete the Store Listing and Content Rating
Set pricing to free
Publish your app. (no worries it will not appear on Google Play)
Then go Google Play Developer Console to "Game Services:
If you not already have that add new game
Link your app “TheGame (DRM-free)” to you game services under “linked apps”
Make sure you set the “Enable anti-piracy” option to “OFF”
Publish your changes
This work with achievement, leaderboards and multiplayer.
The users don’t need to be testers anymore.
Also multiplayer between your DRM-free and your Google Play app is possible.

You may want to check this document- Alternative Distribution Options
As an open platform, Android offers choice. You can distribute your Android apps to users in any way you want, using any distribution approach or combination of approaches that meets your needs. From publishing in an app marketplace to serving your apps from a web site or emailing them directly users, you’re never locked into any particular distribution platform.
Distributing Through an App Marketplace
Distributing Your Apps by Email
Distributing Through a Website
User Opt-In for Apps from Unknown Sources
Android protects users from inadvertent download and install of apps from locations other than Google Play (which is trusted). It blocks such installs until the user opts-in Unknown sources in Settings > Security, shown in image below. Users need to make this configuration change before they download your apps to their devices.
Users must enable the Unknown sources setting before they can install apps not downloaded from Google Play.
Note that some network providers don’t allow users to install applications from unknown sources.
Also you can check this SO question about : What is the fall back plan for devices without Google Play Services, this may add information to ensure your app should run.
Hope it helps!

How to set an Android "hidden" app inside PlayStore?

I have a subscription based web-service that allows you to monitor industrial machines. It supports Raspberry PI and Android.
Right now, the Android APK is downloaded from my website, but I want to put it in PlayStore to be able to push silent updates (most of the devices running it don't have a screen).
My question is how to make it "hidden" in PlayStore ?
I want only people coming with a link from my website to be able to download it. I want it to not appear in searches or in any Store listings (new apps, popular, etc). The only reason to have it in the store is for silent updates that are not possible in any other way.

I don't know if there is an official solution for this, but Apps with only Alpha and Beta apks are always hidden in the Playstore.

Google Play now allows you a new way to distribute internal applications(Google Play Private Channel):
The Google Play Private Channel for Google Apps (the Private Channel)
allows Google Apps domains to distribute internal Android applications
to their users through the Google Play Store.
Here you can find the announce on Google Enterprise Blog and here the instructions to set up this feature.

Is it possible to program an android app that behaves similar to google's Play Store?

Is it possible for an android application to act as a custom market place?
The imagined behavior of the app:
Download XYZ app from the built in Google Play Store
Opening XYZ app yields a UI similar to that seen in the Play Store
The user can navigate through XYZ app and select a custom app supported by XYZ to install on their phone
This custom app would not be located on the Play Store and the .apk would be directly installed from the context of the XYZ app/marketplace to the users phone
I did some research and found alternative Android App Stores such as:
Amazon App Store for Android
GetJar
SLideMe
F-Droid
But is it possible to have an alternative marketplace like this that can be accessed from an app that was downloaded from the Google Play Store? Does google allow this type of practice?
So I suppose the heart of my questions is: Is it possible to create an android application that has permissions to install other 3rd party applications directly to a users phone? (without needing to leverage the Play Store or needing to jailbreak the phone)
Thanks in advance for the answers and insights! And please let me know if anything I'm saying doesn't make sense.
(Extra Credit: I would also be curious to learn about the feasibility of doing this with an iOS app as well)

To add to the other answers, you will have problems with some of Google Play's rules. AFAIK they don't allow your app to direct users to a different store. So you won't be able to keep your app on Google Play store in that case.
EDIT
It is not in the Terms and Conditions, as confirmed by #Cumulo Nimbus.
It is in clause 4.5 of the Developer Distribution Agreement:
4.5 Non-Compete. You may not use the Market to distribute or make available any Product whose primary purpose is to facilitate the
distribution of software applications and games for use on Android
devices outside of the Market.
My interpretation of the above was, and still is, that we can't point people to a different app store.
Kudos to #zmarties for pointing me at the Developer Program Policy which links out to the Developer Distribution Agreement. I knew I'd seen this somewhere...
In terms of iOS, Apple does not allow different app stores unless you jailbreak (iOS-speak for rooting) your phone. They are not as open minded as the Google.

Android apps don't need to be rooted in order to install third party apk's. The use just needs to allow this in their settings. The apps you have listed simply instruct their users to enable this setting.

The best example of a third-party "free as in freedom" app store would be F-Droid

Technically it's possible.
The current Developer Program Policy does not seem to prohibit it either - all they have to say on the matter relates to installing "dangerous products" from outside the play store:
Dangerous Products: We don't allow content that harms, interferes with the operation of, or accesses in an unauthorized manner, networks, servers, or other infrastructure.
Malicious scripts and password phishing scams are also prohibited on Google Play, as are apps that cause users to unknowingly download or install apps from sources outside of Google Play.
Having said that, I can't quickly see any alternative apps stores that are themselves in the Play Store.

Is there any way to download an app beyond Play Store?

I've published an app and I have a theoric doubt. In my google play administrator page, it shows me that my app was downloaded by 18 devices, but when I look at the google analytics, on the same period, I have over 100 devices.
I've looked for other links on Google, but every link that I found redirects to Google Play, so Google Play should count it. Is there any way to download the app from google play, and then, get the apk?
The funniest part is that I didn't spread this app and when I look at the place of the users, it's from India, Sudan, etc..(I'm from Brazil, the other side of the world)..

Yes there is a way. Apk can be downloaded from Google Play and than shared as file somewhere else. These downloaded apks are disributed commonly in these countries like India etc.
For example there is an extension to it for chrome https://lekensteyn.nl/apk-downloader/.
So I would say your Google Analytics data are valid.

Is there any way to download the app from google play, and then, get the apk?
Sure. Copying the APK off of a device is not that difficult. In addition, software pirates bulk download pretty much everything uploaded to the Play Store and make it available from their own sites.