There is an implication here that the back button is a microcosm that represents the differences holistically but it's not. It's a small one-off exception that doesn't even affect the design choices made for the various apps. As I open up app over app on my Android (Audible, Spotify, ChatGPT, etc.), they all have clear in-app "back" functionality where needed. No apps I have installed rely on the Android back button exclusively. Also, none of these apps have any design elements that follow Android's design language.

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.