How do you refill them small bottles?
Refill adapter, you can get it at many places that sell propane stuff... Walmart, Menards, HF, etc. Put the empty small bottle in the freezer for a few hours, screw the adapter into a 20lb bottle, flip it over, screw on the small one, open valve, close after a minute or two. Done, same process for any little bottle, including legal reusable ones. The freezer part is not necessary, but it allows you to get them closer to "full"... regardless, they never get "full" per se, maybe half or a little more at best, not the 80% full like they are new. Flame King makes a refill kit that allows you to get them filled to their safe fill level.
It is actually illegal to refill the little Coleman ones and can be unsafe.
The valves are not really meant for multiple uses.
That is correct, and the whole thing surrounding that is kinda interesting. My refill adapter has color instructions with it that show it in use with a green disposable bottle. Many of the adapters for sale on Amazon and elsewhere specifically advertise that they are for refilling disposable bottles. I am sure like everything, this is part facts and part politics.
I have a 5lb propane bottle that I will eventually start using for everything and transition away from the disposables entirely. I just haven't really figured out where it fits into my camping gear arrangement.