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.

Regardless of configuration, you still need to access the instance from mastodon.mydomain.tld or lemmy.mydomain.tld, but you can configure the Mastodon to use @[email protected] as its identifier by setting WEB_DOMAIN to mydomain.tld.

Mastodon documentation on the topic: https://docs.joinmastodon.org/admin/config/#web_domain