On desktop version of website, once you clicked into the community you can click subscribe button on the right side.

On mobile I see it after I click “Sidebar” button.

Go to https://lemmy.world/communities/listing_type/All/page/1

This page lists communities from other lemmy/kbin servers that federates with the server you use. (server list available in https://lemmy.world/instances)

If you know other community’s url from other server (e.g. https://lemmy.ml/c/asklemmy), copy and paste that to search function and you can browse that community without registering an account on other lemmy/kbin server.

Currently beehaw.org has de-federated with lemmy.world, that means if you paste a community from beehaw.org, the search function might (1) find the community but you won’t see new posts and your posts and comments won’t show up on beehaw server (2) doesn’t find that community.