As someone who mostly uses iOS and occasionally Android, I’ve had difficulty integrating the Android back button into my usage. Because its behavior within apps is defined by devs, I feel like I never have a grip on what to expect when using it (sometimes it’ll close a sheet, sometimes it won’t, etc) and adding inter-app navigation muddles that further. Every time I try I end up enabling the iOS style gesture bar at the bottom.
Convention is to keep it in the top left. All the stock apps will use more or less the same navigation dock up top for back and other functions. Often apps will be rejected for not providing this kind of functionality predictability. This was the hardest thing for me to learn moving from Android to ios as well, but the smoothness of ios trumps all the hardship in my opinion.