Is there a way to set it up so user@example.com can be a lemmy account and also a mastadon account? I seen people using subdomains like user@lemmy.example.com and user@mastadon.example.com is this nessasary? Could u also set up a matrix account with the same user@example.com? If not what woukd be requured to change to make this possible?
You do it via a webfinger. But the service itself has to request a specific resource in order to route it correctly. For example, mastodon and pixelfed ask for the same resource, so both services will end up getting the same service’s account.