I’m commenting here from another instance (feddit.de) where I’m logged in and this community is available at https://feddit.de/c/lemmyworld@lemmy.world
Similarly, you can just access communities from other instances here, e.g. https://lemmy.world/c/asklemmy@lemmy.ml
They are separate. Think of community names like email addresses: john.smith@here.edu is different from john.smith@there.com
Similarly, gaming@lemmy.ml is different from gaming@beehaw.org