I think the best practice (as seen in pretty much every os) is to place the backwards button on the leftmost edge.
The reason for that may have to do with the fact that our culture is used to reason from left to right. That way, what is on the left comes before what is on the right. That is the same reason the arrow itself is facing to the left.
As to the position, it is on the left because: a) that is where you would hope to find it (again, because of left being “previous” to our brains) as well as to b) avoid misinterpretations, as otherwise it may seem like the arrow is pointing to something else. That is why the phone with the arrow on the right has that loop around it.
Overall I think it is the norm because we’ve grown into it and makes sense given we read from left to right.
Hope that helps