Develop Reference

Are AppCompat frameworks and libraries necessary for new Android projects?

I'm just getting into Android development. I'll be using Xamarin.Android. Reading into this, and having toyed around with Android in the past, are AppCompat frameworks/libraries still necessary? I ask this because it seems that from Nov 1, 2019, all new app submissions require at least API Level 28 as the minimum, so what use would it be to include all these bloated appcompat libraries?
Correct me if I'm wrong or mistaken.

#wpa
AppCompat should not be necessary if you are starting a fresh new project. You can start using AndroidX libraries which will provide backwards compatibility. The requirement being mandated starting Nov 1, 2019 is not for the minimumSdk. It is for the targetSdk version. These are different. You can still have your minimumSdk to the lowest you want, but your app should target at least Android 8.0 (API level 26).
Please follow the link below for detailed information.
https://developer.android.com/distribute/best-practices/develop/target-sdk

Google requires set targetSdkVersion api level in 28 for currently apps in the Google Play Store, but if you are starting a new project, surely will start by default withe the latest android version 29.
Feel free of choose the minSdkVersion in your project.
targetSdkVersion: normally the latest android version available (for example API 29/Android 10)
minSdkVersion: the min Android SDK that your application can run on (for example since 21/ Android Lollipop)
androidx
Other hand, it is still necessary to support previous versions if your plan is to reach the majority of users using new features in old versions of android
However, according to android documentation, developers should be start projects using androidx libraries and not use appcompat library anymore.
Artifacts within the androidx namespace comprise the Android Jetpack libraries. Like the Support Library, libraries in theandroidx namespace ship separately from the Android platform and provide backward compatibility across Android releases.
https://developer.android.com/jetpack/androidx
https://developer.android.com/jetpack/androidx/versions/
https://developer.android.com/jetpack/androidx/migrate (if in the future you find an old project that needs to migrate to androidx libraries)

Is it possible to support newer android sdk versions in react-native?

I was following setup guide for android in react-native documentation
and have it up and running correctly using suggested SDK v23
However there are much newer SDK versions available, I would like to set v23 as a minimum SDK version, but use latest one as my main one.
Being new to android I'm not sure on how to achieve this, what steps should I take? I can see option to edit sdk versions in some xml files and can download latest one using Android Studio.
As a side note, what are my limitations of using SDK v23? Will it not work on newer devices, do I loose out on performance etc... ?

According to some sources React Native does support up to API level 25, but there seems to be some disagreement on the issue. So according to official sources, React Native only supports Android API level 16 to level 23.
I haven't used React Native myself, but assuming versioning works the same as native development, an application with a compileSdkVersion lower than the version it is running against will run fine. The only limitations on the application would be that it would be unable to access the new features and APIs introduced by those higher SDK versions.
For example, Android 8.0 (API level 26) introduced a new Picture-in-Picture API, but the method used to enter picture-in-picture mode would not be available in code compiled against API level 23. Some new features may effect performance (usually positively), but an application compiled against API level 23 won't perform any worse on on a device running on API 26 than one running on API 23.

Xamarin Targetframeworkversion

I am trying to make a basic Xamarin.forms (pcl) application (in Visual studio 2017) and i want to Target Lolipop (5.0.0, api level 21).
So i made a basic Xamarin solution and i adjusted the Android properties all to Android 5.0 lollipop
Compile using Android version "Android 5.0 (Lollipop)"
Minimum Android to target "Android 5.0 (API Level 21 - Lollipop)"
Target Android Version "Android 5.0 (API Level 21 - Lollipop)"
Unfortunately in my warning list i get the following
Severity Code Description Project File Line Suppression State
Warning The $(TargetFrameworkVersion) for Xamarin.Forms.Platform.dll (v7.0) is greater than the $(TargetFrameworkVersion) for your project (v5.0). You need to increase the $(TargetFrameworkVersion) for your project. Prolang.Android C:\Projects\Prolang\Prolang\Prolang.Android\Prolang.Android.csproj
If i google for this error most people say to indeed update your android project to use the latest Targetframework version. But in my case i really want to only be able to call Api calls that are available to level 21 aka Android 5.0 Lollipop. As in i want the compile time safety net that i only use Api Calls from Api level 21.
To me it seems like i have to downgrade my xamarin.Forms dll to a version that is made specifically for android lollipop since the one i use right now targets a higher version.
So should i downgrade my xamarin.forms dll to achieve this and get rid of the error? And if i downgrade to what version do i need to downgrade so i target Android 5.0 Lollipop? This information is all pretty unclear to me.

Xamarin.Forms is designed using the Android's support libraries so it will run on a wide range of APIs without any additional actions from the developer.
If you are using native APIs via Xamarin.Android (assumably via Forms' Dependency Service) then yes you would have to be aware of the API level of the method calls.
That said, since Xamarin.Forms uses the com.android.support.* libraries and you would have to have to a Forms versions that uses com.android.support:design:21.x.x and the associated Android build tools for API 21.
Now you can review the package dependancies and you will see that your choices are Forms' is 1.4
`1.4.0.6336-pre1` to `1.4.4.6449`
Note: v1.5.x moved to version 22 of the Android support libraries
IMHO using Forms's 1.4 would a bad mistake due to its age and the mountain of improvements and bugs fixes since 2015, you are so much better off:
Using the latest Forms version
Set the Framework version to latest (7.1)
Set the Target Android version to Android 5.0 (API Level 21)
Set the Minimum Android version to whatever you need...
Forgo the compile time API checks
Check runtime API issues via API-21 devices and emulators

does fragment manager works in android API 8

I want to know whether the Fragment Manager works in Android API 8 ? I am using the project to compile with Android API 8.
if yes tell me how to use the fragment in lower devices or suggest any tutorial links.

The Android Support Library backports many newer features to older Android versions. In the case of Fragments, they are supported via the support library back through Android 1.6 (API 4) as explained in the features overview. The setup instructions go through everything you need to include the Support Library in your application.

Which SDK should I use to build the project with Support Library?

I'm not sure which SDK version should I use to build the project if I added a Support Library. Shouldn't I build it with 2.3.3 for example?

It doesn't really matter which version of the Platform you are using with the Support Library.
The Support Library is designed to back-port some important new features from Android (like Fragments), so they are supported in older versions of the OS (back to 1.6).
Bottom line, you can use any version of the SDK with the support library (all the way back to 1.6).

You can use the support library in API level 4 and above, so you can build it against pretty much whatever version your other APIs require.