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).

This. Prometheus is for metrics, Loki is for logs. I’ve not come across Perses before, but it looks like it does a similar job to Grafana which integrates really well with Prometheus and Loki (and is produced by the same organisation as both of them).