All those services use a protocol named WebFinger that allows for having the servers in different domains but showing the users as being in a main domain. For example, my self-hosted Synapse server is at matrix.domain.tld, but my users are all like [email protected].

Unfortunately I don’t really know how to set it all up manually, I just let Yumohost take care of managing it.

Matrix (or even easier XMPP) can coexist on the same domain with Lemmy or Mastodon, but with two ActivityPub services that is AFAIK not possible.