i am a certified autobody tech. i install thousands of fasteners monthly.

plastic rivets are NOT REUSABLE. you have to replace with another rivet or a comparable push in fastener. they use a special rivet gun to install. a lot of times the inner plastic pin breaks when installing. they are somewhat a pain in the rear to install sometimes. so most times i just cut and replace with a push in fastener when i can.

the plastic push in fasteners, like the "christmass tree" earlier pic of one, are rather difficult to remove and reinstall without damaging the part they are attached to. they twist the fender tabs holding them in when you try to save them. on them pushed into a solid part i use a bent end long nose pliers and pull them out.
on fenders i just cut with a side cutters and replace.

the push pin fasteners are easy to remove and replace. just pull the inner pin out with a side cutters or comparable tool and the outer pin usually falls out. sometimes a bent end pick works great to remove the inner pin.

in most body shops have a bin with extra fasteners. i just go into the bin and find another comparable sized one for a replacement part.