Functions can include logic that could check for the presence of

public.users

and

user.auth_id

or whatever is needed and then do the right thing. Right?

True but if we want to copy Firestore it lets you access any document in any collection at read/write time to help you build your rules:

allow: read: if (get(/databases/$(database)/documents/users/$(resource.id)).data.auth_id)

If we copied this for Supabase it would mean you can read a table like

public.users

or if my app stores users in another place like

public.profiles

etc. it would be up to me