That person is using systemd-resolved. You can run both NetworkManager and systemd-resolved, but you need to set it up in a specific way to prevent problems from occurring if you do. I think it is more common to use one or the other.
If you are not sure, check them both to see what you have running:
systemctl status NetworkManager
systemctl status systemd-resolved
If systemd-resolved is not running, you should remove the symlink.
sudo unlink /etc/resolv.conf
Then restart NetworkManager.
sudo systemctl restart NetworkManager
That should repopulate /etc/resolv.conf; you can check like this:
cat /etc/resolv.conf
Test if that has restored your internet connectivity.
ping archlinux.org