How did I create the Android wallet? Well, I can't answer that in a few sentences. You must know how to program in Java/Kotlin. You must have an understanding of Android. Go grab any decent Android book and read it. Then go look through the Bitcoin codebase and study their code. Eventually with enough experience you will be able to program any Android app to do what ever you want! It is very difficult to do if you have never programed before. Good luck, let Google be your friend!