I know the title starts with a capital s but this is so that Drupal sorts it better! Technically it should be "systemd" as that is the official name.
systemd is the standard service manager on Linux
systemctl is how you start stuff and you just need a simple service file, not all the start/stop boiler plate code as in shell scripts from before
systemctl start myapp.service
It automatically creates a cgroup
It allows templates by using @ in the service file
With socket activation, systemd listens on the port and fires your app when a request comes in
systemd
systemd/systemd: systemd System and Service Manager