What does Android rooting imply in terms of user accounts?

I've read dozens of articles about rooting Android, all explaining how and why, but couldn't find one explaining what really happens to the user.
I don't have an Android device, I'm writing in a Linux user perspective.

Which one (if any) of these scenarios is true:

1. Once rooted you always log in as root. It's the end of individual users.




2. Once rooted all user accounts still exist as before but now have full root privileges. Created files still get the user:group of whoever is logged in.
   2.1. If the above is true, can it be enabled for some users and disabled for others?




3. Rooting is nothing more than having access to a 'sudo'-like command. You can use it when and only when you decide. All user accounts are unchanged.
   3.1 If the above is true, can it be enabled for some users and disabled for others? Is there a 'sudoers'?




4. If none of the above, what?