How do I change the default runlevel in Feodra 15?

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