I think you are getting confused there, the doc you are linking to are for linking an external application to your Nextcloud account, whereas your diagram seems to be connecting Nextcloud to an external IDP.

You may be wanting : https://apps.nextcloud.com/apps/oidc_login or https://apps.nextcloud.com/apps/user_saml

https://oauth.net/articles/authentication/

That aside, why is nextcloud asking for scopes from remote API in the diagram? What is drawn on the diagram has little to do with OAuth scopes, but rather looks like an attempt to wrap ACL repository access into a new vocabulary.

Scopes issued by the OAuth authorization server can be hidden entirely. The issuer doesn’t hold any obligation to share them with authorized party since they are dedicated for internal use and can be propagated via invisible or opaque means.

I really can’t figure out what’s going on with that diagram.

OpenID via keycloak