Local communities should simply be displayed to you as an option to subscribe to. These are easy. However finding communities out in the “fediverse” is a different story.

The “threadiverse” (part of the fediverse) is a bit hard to navigate and find communities you might want to join. Usually this process would be to navigate to a specific instance and click their “communities” link to see all the communities that the instance has and then manually grab their fediverse ID, search for it in this instance, and the subscribe to the community.

This is quite cumbersome. And there are a few shortcuts that can help us out here, especially in identifying multiple communities on different instance simultaneously.

(in no particular order)
https://browse.feddit.de/
https://browse.toast.ooo/communities
https://lemmyverse.net/ (personal favorite at the moment)

Are all community browsers that allow you to search for particular keywords. From here you can get the fediverse ID (!<community>@<instance>), search it in this instance, and then subscribe to that community.

Searching a community may not yield a result immediately on the search screen. That is normal. Even without results it will trigger this instance to reach out and find that content. You should see the community you searched for (if it actually exists) show up in the Communities page as subscribe-able. You will also see communities that others on this instance have searched before (not necessarily subscribed to).