Hi, I'm figuring this out too. Veterans, please feel free to correct me!
If you create a community on kbin, it does not prevent a community with the same name from being created on another site. We have a 'music' community here (kbin.social/m/music), but lemmy.world has a 'music' community too (lemmy.world/c/music).
(kbin calls them /m/agazines, lemmy calls 'em /c/ommunities, if you're wondering about the 'm' vs 'c' in the URLs lol.)
Yes! Because Fediverse magic, you can actually interact with the lemmy.world 'music' community without leaving kbin.social. Try it out from kbin.social/m/music@lemmy.world.
You may notice this @[object]@[place] pattern a fair bit.
For example, our own 'music' community is @music@kbin.social. The lemmy.world 'music' community is @music@lemmy.world. Your account is @Spider-Man@kbin.social. See if you can spot users or threads that are from other sites! They're probably already in front of you!
As a second experiment, see if you can visit the music community from beehaw.org, without leaving kbin.social. They have one too, but you don't need a beehaw.org account to participate there!