When i first set up my linux desktop it asked me to add a user account . After then i was learning linux commands using this account. I then add some more users using "useradd" command. Now i eager to known that is there any difference between first user account and those i created using 'useradd' command and how can i figure out those difference.
/etc/group for whether the first account is in some groups that the second is not.
/etc/sudoers to see if it's individually mentioned as permitted to use
sudo, and either add an entry for the second user if so, or enable a group sudo permission for a group that your new account will be in, conventionally
Also, you mentioned "you can't change some files" with it. Check the owner and group of those files (in
ls -l), and if you want both users to have equal access then set permissions on this file to ones allowing group the same as owner and have the files' group set to one that both users are in.