This is a really good question.
For micro-blogging, the only one of those that's internal to your service, is "boosts". The equivalent for lemmy, would be cross-posts on communities you already follow, where the url field is either your local copy, or the federated link. Someone would x-post a link from another community, you'd see it, then subscribe to it.
A better answer to your question tho, would be a global community discovery service. Someone could either write one, and @nutomic@lemmy.ml has also written a lemmy-instance-crawler for join-lemmy.org to do the same. We could also potentially expand what join-lemmy.org shows, to list the most popular communities globally, and for each instance.
Since communities aren't tagged, the only real way to look through them would be by popularity tho. But it would at least be a centralized place to see every known community on every known instance.