Word of mouth, (social) networking and search functionality. And I think Mastodon as it's focused on people, and Lemmy which is focused on topics, require different handling. There are a few articles out there outlining that Stephen Fry and a few other famous people are on Mastodon. I'm not sure how someone would recommend me my favorite blogger and YouTube stars though, without me searching or clicking on a link next to their content... I think Lemmy is pretty alright. You can enter a term in the search bar and it'll show you the communities with that in the name. Or related posts. Also there are a few websites with listings and directories of communities. The UI could be better though, for example show other communities on cross-posts or include related communities in the sidebar...

And the "proper" way to do it would be to implement what commercial platforms do. Track the users, learn about their interests, have an "algorithm" that gives good recommendations.