Earlier quoted context omitted.
In inconsistent places. In many UIs they're on the top left. In the browser it's on the bottom left? So weird.
You can however consistently swipe from the left side of the screen to go back. There are a couple of apps which I suspect were not written by iOS users where this functionality is broken (such as Valve’s Steam iOS app), but those are far in the minority.
I used my coworker's iPhone yesterday, opening a link from a text message, and couldn't figure out how to go back to the message. Swiping in from the left side went back... within the browser, which had no back history. I had to go to the home screen and open text messages again.
On Android, you just hit the back button and it goes back to the previous app in the stack if you launched one app from another.