Built-in read-only user still has edit rights, how to manage user permissions?

where should I look for Users Guide plz ? I have already crawled through the doc, I have my instance up and running but can't find knowledge on HowTo-Manage UsersInThisPattern

like for my first try : having a built-in user admin with all rights (easy, out of the box), and a second built-in user, that can only Read on the built-in application. I have created a Read-Only Permission with my second user as a subusers, but he still has the right to edit.

I hope my expectations are clear enough for anyone to help 🙂

use a non-built-in user to do this

built-in users are like "root" in linux