I've just installed Ubuntu and my user is a superuser which can execute commands with sudo. So I thought this user is added in /etc/sudoers file. But it seems it's not there:

#

# This file MUST be edited with the 'visudo' command as root.

#

# Please consider adding local content in /etc/sudoers.d/ instead of

# directly modifying this file.

#

# See the man page for details on how to write a sudoers file.

#

Defaults        env_reset

Defaults        mail_badpass

Defaults        secure_path="/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/snap/bin"



# Host alias specification



# User alias specification



# Cmnd alias specification



# User privilege specification

root    ALL=(ALL:ALL) ALL



# Members of the admin group may gain root privileges

%admin ALL=(ALL) ALL



# Allow members of group sudo to execute any command

%sudo   ALL=(ALL:ALL) ALL



# See sudoers(5) for more information on "#include" directives:



#includedir /etc/sudoers.d

The user info:

uid=1000(someuser) gid=1000(someuser) groups=1000(someuser),4(adm),24(cdrom),27(sudo),

30(dip),46(plugdev),113(lpadmin),128(sambashare)

How did I get superuser privileges?