Instances can de-federate each other so you can't see communities. Beehaw just de-federated us due to the large influx of users containing some people trolling their communities. The moderation tools are not great right now and so rather than dealing with the influx of trolls they de-federated this instance.

So you can't see Beehaw communities and their users cannot see your posts if you post on another instance's community.

This lets instances decide who they want to associate with and is a good thing for the long-term health of social media. You don't want to be a part of instances that associate with terrorist instances or other shady things, de-federation is the tool to keep that from happening.

In general, any instance is as good as any other but as time goes on there may be a few different 'islands' of instances that are functionally separate from each other and you'll either choose one or use multiple accounts to swap between them.