SYSTEM is arguably not a user, because a user has things like a password, groups, and other user properties. SYSTEM on the other hand is a built in low level security context which cannot be removed, altered, or shared across a network. You cannot login as SYSTEM, but processes can run as SYSTEM. It has a static SID on all Windows systems as S-1-5.
Some of the processes that run as SYSTEM in SYSTEM: ntoskml (Windows Kernel), all drivers, and all interrupt handlers. A few key services (not all, and fewer every release).
Literally it doesn't get higher than NT AUTHORITY\SYSTEM on Windows. The administrator account no longer exists on most Windows installations at all, and the administrators group is just that, a group.
It is like root on UNIX, if the root account had been configured to disable interactive login and to allow sudo only.
PS - SYSTEM is sometimes called NT AUTHORITY in documentation. I've seen it called both, I've also seen it called "NT AUTHORITY\SYSTEM." I believe the NT AUTHORITY is referring to the lowest level of permissions within the OS, and SYSTEM is at that level/root of the permissions table. So really NT AUTHORITY and SYSTEM are equivalent.