I’ve found that similar communities on different instances can have VERY different experiences in terms of the community and attitude. I know I’m generalizing, but I feel a lot of Lemmy.world instances are more negative and hostile than similar instances on beehaw.org. Personally, I’d rather they stay separate. That way I can be more precise on dialing in the experience I want.
You sign up on (or create) an instance that defederates from them.