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

I much prefer “Move slowly and fix things” (I so wish I had thought of that myself but can’t remember where I saw it).