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.