Reachability and a right handed keyboard. And some shifting of the device in my hand as required.
No, I cannot reach "all four corners" if I do not move my hand at
all on the back (and I do probably have smaller hands than you, assuming you are an adult male) but I don't find it hard to sort of re-position my fingers and hand/grip as I go if I need to reach a corner. I usually use it right handed and can reach the upper left corner easy enough if I rest the side of the phone in my palm for example, with my fingers sort of curled to grip onto back midway. I almost always use it one handed, including while doing other things (on elliptical, walking cat, emptying dishwasher, etc

)
I come from a 5s so it took me a few days to really get it down, but now I have no issues making adjustments as necessary as I use it one handed for one handed use. Definitely not like using a Plus for me at all, since that was too wide for me to really get any sort of grip on it that allowed one handed use.
I do have a Spigen Slim Armor case that is a bit "rubberized" and rounded on sides that helps, for me anyway, as it is not slippery at all and is comfortable to hold. I can even use the little opening on back (at the Apple) for my grip at times.