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?
Each fediverse service is different. Matrix has it’s own webfinger configuration, as does diaspora, and mastodon. However, it looks like all ActivityPub services use the same webfinger configuration, which means that this method doesn’t work for donations trying to reference user@domain.tld for activitypub. I would presume that the first AP service you have listed in the response is going to be the account that each AP service will federate with.
Here’s a little bit about what I’m talking about:
https://www.hanselman.com/blog/use-your-own-user-domain-for-mastodon-discoverability-with-the-webfinger-protocol-without-hosting-a-server
https://github.com/pixelfed/pixelfed/issues/3563