DiscourseConnect is Discourse’s implementation of SSO. It doesn’t use a standard protocol.
If you don’t mind looking at PHP code, there’s an example implementation here: https://github.com/discourse/wp-discourse/blob/main/lib/sso-provider/discourse-sso.php.
Yeah, that’s not going to work. If you have an OAuth2 provider that you’d like to use for authenticating users, have a look at the Discourse OAuth2 Basic plugin.