I'm new too, but here's what I've learned in the last week:
You're a user of and logged into @beehaw.org. This post (and the community it was posted to) exists on the @lemmy.world instance. You can see and post to it from your beehaw.org instance, because @lemmy.world also exists in the Fediverse.
My instance is @lemmy.world, so this community/post is "local" to my instance, but in practice that's not super important. All that tells you is where I enter the fediverse, from there we're able to see and post in communities from across instances. For example, I can see communities/posts from @beehaw.org, where you are. I am subbed to a few communities there.
It's possible that a community like /c/games exists on @beehaw.org and on @lemmy.world. You would see them as games@beehaw.org and games@lemmy.world, and they are separate communities (despite having the same community name) so you can sub to one or both. OP is basically suggesting a feature to group (for example) games@beehaw.org and games@lemmy.world so that it just looks like one big community.
More experienced Lemmy and Fediversers, please correct any errors I may have made it this post!