Each group/sub on Lemmy/Piefed/etc. has an address, like subname@lemmy.website. On Mastodon, you just @ the sub’s handle, and you can follow the handle as if it were a user and have everything posted to the sub reshared to you.

Lemmy doesn’t let you follow individual user accounte, though, so you can’t follow Mastodon users. Just groups. But there are lots of things that are groups. PeerTube channels, for instace.

You just mention the community’s handle like you would another user, and that creates the thread. Hence the @fediverse. Behind the scenes, the community then does the equivalent of a “boost” to push the thread to everyone who is following the community. If I recall correctly.