One doesn’t need admin rights, if app.service is saved in ~/.config/systemd/user/.
In autostart-app.service, I have
[Service]
Type=Oneshot
ExecStart=/home/nemo/.config/systemd/scripts/autostart-apps
and autostart-apps-script runs each time I restart my phone.
And running systemctl --user enable autostart-apps doesn’t require admin rights either.