systemd is a proocess management solution to manage running processes (daemons) on *nix based systems. It is used to set what services run in the background, how they start up, what resources they have access to and more.
These daemons are controlled via .service files. We can look at the easel-driver systemd file at https://github.com/inventables/iris/blob/master/build/linux/easel-driver.service to see an example.
Service files live in /etc/systemd/system or /etc/systemd/user folders and this is where we put easel-driver into https://github.com/inventables/iris/blob/7f1c7bf96a59d46f030b3b3b3d32a1df5797d918/build/ci_build_linux.sh#L12
To control deamons we use systemctl utility https://github.com/inventables/iris/blob/7f1c7bf96a59d46f030b3b3b3d32a1df5797d918/build/linux/DEBIAN/postinst#L30
# on a local *nix box
sudo systemctl status easel-driver