It’s possible with OpenID. All you need is extract domain from login id and get provider configuration at https:///.well-known/openid-configuration. Problem is that if it’s down you can’t login into same account using another provider.

Technically same way as google or apple SSO works just fixed list of domains. Another problem with using whatever domain user supplied is that some domains are more trusted than others, or rather owners. I don’t mean from privacy perspective I mean stability and security. It’s much easier to hijack domain of small server which struggles to pay for domain name consistently than google.

If Fediverse want to use OpenID for login someone have to vet trusted auth providers.

Otherwise we need someway to federate user logins. Maybe literally have replicated users credentials across instances. Then all federed instances can authenticate users from any federed instance.