Changing default target (aka runlevel) permanently - UBUNTU 18.04

I know I can change the default target with this command :

systemctl set-default multi-user.target 

However, there is a line in /etc/default/grub with a text :

GRUB_CMDLINE_LINUX=""

Where if you put a number between quotation marks (I’ve read), it will set a default target(runlevel).

My question is which way should I set default target and does putting a number in a grub file affect systemd state ?