There's nothing special from architecture perspective. Identities are necessary for login and for some tasks related to outgoing messages to user: thus, authent

There's nothing special from architecture perspective. Identities are necessary for login and for some tasks related to outgoing messages to user: thus, authentication (OAuth2-based server) works with identities and during the login finds the related account and grants necessary permissions for it. Resources requiring personalization (information from account), will have the account id. If they need to contact the user, they'll request message delivery by this account id - respective service will figure out, which identity has to be used for this delivery.