Q. How do I login as superuser / root user in a Linux operating systems?

A. The superuser, or root, is a special user account used for system administration.
Linux superuser

In Linux and Unix like computer operating systems, root is the conventional name of the user who has all rights or permissions (to all files and programs) in all modes (single- or multi-user).
The root user can do many things an ordinary user cannot, such as changing the ownership of files and binding to ports numbered below 1024. The etymology of the term may be that root is the only user account with permission to modify the root directory of a Unix system.
Superuser login - How to become Superuser in Linux

Under Linux (and other Unixish oses) you use command called su. It is used is used to become another user during a login session or to login as super user. If Invoked without a username, su defaults to becoming the super user. It is highly recommend that you use argument - to su command. It is used to provide an environment similar to what the user root would expect had the user logged in directly. Type su command as follows:
$ su