Answer 1

After reading the first answer in this thread
https://www.quora.com/Can-I-develop-Android-apps-using-C++?top_ans=218152345
I have come to the conclusion that apps are usually not entirely made in C/C++, but some amount of C++ can be mixed in the apps

These might help
https://developer.android.com/studio/projects/add-native-code
https://devblogs.microsoft.com/cppblog/android-and-ios-development-with-c-in-visual-studio/

Mostly Android apps were developed in Java (still are), but Google released Kotlin and is now promoting that for Android development. But it doesn't really matter whether you use Java or Kotlin. Xamarin is a framework which can be used to make apps for ANY platform (Android, iOS, or Windows) using a single .NET codebase. React Native is also getting popular for making cross-platform apps.

As for sources, I don't think this will disappoint
https://developer.android.com/training/basics/firstapp