A service can consist of two files - the .sh script to run, and the .service file which describes its run conditions.
The .service file goes in /etc/systemd/system. The scripts themselves might be best placed in $HOME/.local/bin.
Example - tracker.service
[Unit] Description=Tracker [Service] Type=simple ExecStart=/path/to/script [Install] WantedBy=multi-user.target
After making the new service, systemd requires reloading:
sudo systemctl daemon-reload
simple - the service cannot be called on by others. It runs on repeat.
oneshot - the service executes once, then stops.