Understand command sudo su -

Why if I typed the three commands sudo su - with my user after a passwords is ask, I became root user ?

sudo onlfy work if the user is permit to use this app; from /etc/sudoers file.

su Invoked without a username, su defaults to becoming the superuser.
 
- The optional argument - may be used to provide an environment similar to what the user would expect had the user logged in directly.


man pages

sudo, sudoedit ---> execute a command as another user.

su ---> change user ID or become superuser