"subreddits" are communities. You can find them with the search field from your instance, or using the community browser.

Similarly to how on reddit subreddits are noted with a r/, here communities are noted as c/. So, let's say you wanna join c/angryupvote. You search for angry upvote in the communities you are currently subscribed to (bottom row buttons, second button from the left) and it may return a few different communities on several different instances (servers). You pick the ones thay you wanna join and that's that. The only difference is how these communities are linked. On reddit, it was pretty simple: r/subredditname. Here, since the newtwork is federated, you also have to note the server on which the community (subreddit) resides: somelemmyserver.com/c/communityname.