A systemd service is an .ini file in /etc/systemd/system/. The minimum to run a command at startup:

# /etc/systemd/system/myapp.service
[Unit]
Description=My app
After=network-online.target

[Service]
ExecStart=/usr/local/bin/myapp
Restart=on-failure

[Install]
WantedBy=multi-user.target

After creating or editing, reload and enable:

sudo systemctl daemon-reload
sudo systemctl enable --now myapp.service

enable creates the link that starts it at boot (via WantedBy); --now starts it right away. Type=oneshot + RemainAfterExit=yes in [Service] is for scripts that bring something up and exit (e.g. docker compose up -d) but should count as “active”.