They can’t… in android the OS defines the navigation. Back works everywhere. You can do wierd stuff to make back do silly things and go to the wrong screen, but that takes a bit of effort.
In iOS, so far I’ve seen… swipe left, swipe right, swipe from the bottom, click ‘back on a button on the left’, tap on the screen to bring up a button then click that…
Some actions are impossible… you click on a link in mail to see for example a tracking number… for me a daily occurrence. There is no back gesture available, you have to go back to the home screen and restart the mail app, which is utterly stupid especially when you have to do it multiple times.
On android it’s simple. Want to go back? Wiggle your right thumb. Done.