Makes sense!

Where does Debian fit

Can someone explain why global flatpaks are installed in /var/lib and not /opt or /usr/local?

Fantastic, Linux needs tools like this for users who need to make occasional changes and inspect systemd.

For inspiration, I would suggest you also look at LaunchControl, which is a GUI tool to administer launchd on macOS.

With the Reboot Into Selected, will this modify the next boot only without changing the default?

You still have to come up with the explanation of what started life in the first place!