The app doesn't decide which community to post in; you do. When you pick a community for the post to go to, you might pick [email protected], in which case it becomes hosted by the beehaw.org server. Or you might pick [email protected], in which case it's hosted by the lemmy.world server. If you vote or comment on the post that's on beehaw.org, the vote/comment is hosted there as well, and likewise for lemmy.world.

Perhaps the important point is that [email protected] and [email protected] are different communities, just like how [email protected] and [email protected] are different email addresses, even though they're both Neo.