Kinda the opposite - Firestore documents don't inherently have a userId of the owner. Rather, they just have arbitrary fields.

Making all your documents have a `owner_uid` or `userId` field is just a convention they recommend, because it helps you write rules.

So rather: they have no default system for handling documents that can only be accessed by a given user, but rather you have to construct it using `firestore.rules`, and you end up with something oddly default-insecure.