when I use the web version, and I click new, it does say “post or community”. I’m not sure yet which server that’s referring to.

I'm pretty sure that's whatever instance you're logged in to. feddit.uk, in your case. I've never done it, but just creating a community's pretty easy -- the hard part is finding mods, advertising it, encouraging regular posting and comments, etc. If you ever decide to create a community, check out the weekly thread "how are you doing with your communities?" on !fedigrow@lemmy.zip for tips and encouragement.