how the hell do you relax when you have a cock going in
No one ever told you? Yikes. It's simple.
You relax the rest of your body, take a deep breath or two, and at the same time in your anal region, you slightly "bear down" or "push out" like you are slightly trying to crap. This makes your hole more open.
If my description makes no sense, practice at home on a dildo until it does. BTW, it is also why you (as a bottom) need to clean out before getting fucked. Because most people never want any actual crap to come out, as the bottom pushes his hole open to the cock.
If you have a hairy hole, it helps to shave or buzz-trim the area immediately around it. For me as a hairy bottom: my "hole hairs" being dragged/pulled in contact with the top's dick is quite uncomfortable; especially if he's in a condom. Conversely, a smooth hole is comfortable for us both. A smooth hole plus barebacking (and PrEP of course) is bliss.
Old gay proverb - "If you don't like anal sex, it's because you've never had good anal sex." I think that applies equally for the top & bottom roles.