What is the effect of
chown nobody:nogroup on a single file or directory and what is the effect on files within a directory?
chown is used to change the ownership of files and folders.
Your command just changes the owner user and group to nobody and nogroup. Both of these entities are normal objects within the system.
You can verify this by running
cat /etc/group | grep nogroup and
cat /etc/passwd | grep nobody respectively.
It probably doesn't make a lot of sense without also using
chmod to change the actual permissions.