The back button works at its best when it closes an app and it restores the one that opened the previous one. There could be no in app back button to do that. However the back button used to be an always available hardware button or a touch one on the bottom bezel. It's optionally visible now (I'm on an old Android phone so I'm not up to date with the latest OS) or a gesture, right? So app designers must design as if it does not exist.

I agree. Design as if it doesn't exist incidentally is also how they design for iOS where it doesn't exist.