I read that title as “Is there a FOSS alternative to apps like TikTok?”

Elon: “These geniuses will be of great use elsewhere”

The geniuses are hired by Meta to build a Twitter clone.

Elon: “Not like that!”

You don’t get federated automatically. On your own instance you will only see posts in communities that you subscribe to. What I did was set up a “bot” user that I used to subscribe to a bunch of communities that I might find interesting when just browsing the All feed but I didn’t want in my personal Subscribed feed. There’s a few tools that will automatically subscribe a bot user on your instance to all the top communities but I found just manually going through https://lemmyverse.net/communities and clicking subscribe as that bot user worked for me.