You join one, you can see, post on, comment on, and subscribe to communities on any server it is federated with. Beehaw.org defederated itself from sh.itjustworksl and lemmy.world a couple weeks ago which is making it confusing for some people. If you are on beehaw.org you can still see old posts on those two from when they were federated, but nothing new and you're comments on those old posts won't be posted for other servers to see, and vica versa for sh.itjustworks and lemmy.world users.

You can make an account on beehaw as well if you want, but it gets a bit confusing as to which one you are logged into sometimes.

As a lemmy.world user I type "lemmy.world" into my address bar and do all my browsing from there. You can change your page to look at "all" instead of "local" and see content from all instances it is federated with. Or you can go to "communities" and filter by "all" and see all communities from any instance it is federated with. (As long as one of the lemmy.world users has visited that instance at one point)

No reason to make multiple accounts unless you really want some content from a defederated instance, like beehaw.