I see Postgres etc as the builder. Supabase is more like the realtor; a middle man extracting profits and complicating the situation.

Does Postgres talk OpenID connect directly? Does it integrate SAML easily?

Oh you still have to build the auth system yourself? Well maybe a realtor does sound good now.

No, but most serious major stacks have officially supported or popular maintained libraries which handle all of this out of the box already.