Does RingCentral have the Superuser concept ?

Yes Super user or super admin do exists. The "Super Admin" or "Super User" is the default System Extension assigned to your company when you joined .

Have you gone through these details?:

https://www.ringcentral.com/office/features/roles-and-permissions/overview.html

Which superuser access are your referring to? If you are referring to the admin where other users on the platform can be controlled then yes, there is an option for that - but I dont think you get root access or something because that's entirely on the machine you run your post and get requests from. You can go through the above link shared by Anirban for more info on that