Hey
You can’t actually see or add fields to the built-in users tables. But you cann extend it by creating your own collection and connecting the userID from the currently logged in user to a user_id field in your own collection. This way you add fields for anything, like profiles and more connected to the users functionality in WIX.
Use the built in users system because you can use settings in pages and apps that uses this functions so creating your own is not recommended.
Does this answer your question? If not I will try to help you more.