In Supabase we use a separate Auth server [0]. This stores the user in an `auth` schema, and these users can login to receive a JWT. Inside the JWT is a "role", which is, in fact, a PostgreSQL role ("authenticated") that has certain grants associated to it, and the user ID (a UUID).
Inside your RLS Policies you can use anything stored inside the JWT. My cofounder made a video [1] on this which is quite concise. Our way of handling this is just an extension of the PostgREST Auth recommendations: https://postgrest.org/en/v9.0/auth.html