How do I change the default runlevel?
systemd uses symlinks to point to the default runlevel. You have to delete the existing symlink first before creating a new one
rm /etc/systemd/system/default.target
Switch to runlevel 3 by default
ln -sf /lib/systemd/system/multi-user.target /etc/systemd/system/default.target
Switch to runlevel 5 by default
ln -sf /lib/systemd/system/graphical.target /etc/systemd/system/default.target
systemd does not use /etc/inittab file.
How do I know the current run level?
runlevel command still works with systemd. You can continue using that however runlevels is a legacy concept in systemd and is emulated via 'targets' and multiple targets can be active at the same time. So the equivalent in systemd terms is
systemctl list-units --type=target