OAuth2 is a suite of protocols and standards you can use to build an auth system from people who have been working on auth systems for a while. it isn't strictly designed for federated auth, and arguably is targeted at internal systems because there are so many parts are "you need a system that does this, but how is up to you"

OIDC builds on OAuth2 to fill in those gaps with prescribed details and add additional systems necessary for a workable federated auth system. it _should_ be possible to use it entirely provider-agnostic fashion, but in practice it's like any complex protocol with optional parts in that implementations have incorrect bits or idiosyncrasies (why does Azure's provider sometimes serve public keys it doesn't actually use to sign tokens in some modes? god only knows) and you'll probably only provide integrations for a subset of providers, not just let anyone with a discovery link show up and use it.