I'm not using Reddit in any capacity since they have started giving their content for LLM training, so I can't help you with that, but looking at 4-5 third party applications right now, they all have a left arrow at top left to go back.

They all are very different applications and have very different designs, yet the arrow is there.

To be honest, I baffled at your question for a second or so, because I never thought about that, yet the method is so universal that I was not thinking about it at all.

There is a common way to go back (swipe from the left edge of the screen). Some apps just don't integrate well and ignore the platform patterns.

I believe swiping from left to right is common for both Android and iOS.

Android has both a swipe gesture or a widget that simulates buttons that used to be at the bottom of the screen

...and iOS has both an arrow at the top left and a swipe gesture. I can't see how they are that different.