Yeah, I know there aren’t actually run levels anymore. The systemd equivalent of running init 3 to boot into console mode is

systemctl isolate multi-user.target

And the equivalent of running init 5 to boot into GUI mode is

systemctl isolate graphical.target

This is a one-time thing, not a config change. If you want to permanently switch to console mode you’d use systemctl set-default multi-user.target