I found out more about "system" addons. There are different types of addons: app-profile, app-system-addons, app-system-defaults, app-global, app-system-local, app-system-share, app-system-user, app-temporary.

The "app-system-defaults" (in my case Pocket & Loop/Hello) don't seem to be shown to the user, but you can find them in the extensions.xpiState config.

https://hg.mozilla.org/integration/fx-team/file/5f2f4297e6bd...