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:


Once rooted you always log in as root. It's the end of individual users.
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?
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'?
If none of the above, what?
As you might have expected from the "Linux user perspective", item #3 is the correct answer. With a little adjustments: apps that come with "root features" can use the "root functionality" as well. When they try it the first time, you'll have to approve that, though:

Basically, a "clean rooting" brings in two things: a su binary and a kind of "supervisor app" (SuperUser/SuperSU). The former you can use e.g. at the shell to obtain root access – and the latter controls whom superuser powers are granted. The first time an app tries to obtain those powers, you will see a popup on the device asking you whether you confirm or deny. A check-box lets you store your decision permanently, if you wish; leaving that unchecked, the popup will re-appear the next time.