Passage: The command has the same effect as the Unix command id -un. On Unix-like operating systems, the output of the command is slightly different from $USER because whoami outputs the username that the user is working under, whereas $USER outputs the username that was used to log in. For example, if the user logged in as John and su into root, whoami displays root and echo $USER displays John. This is because the su command does not invoke a login shell by default.

Question: What does $USER use as far as the person username?
the username that was used to log in