When visible (via devtools.aboutdebugging.showHiddenAddons=true), the system addons and hidden extensions are mixed in the regular extensions category.
It could be nicer to have subcategories for each of them:
> Temporary Extensions
> Hidden Extensions
> System Addons
Although compared to the old about:debugging, since we also have tabs, workers and processes in the same UI, maybe it starts feeling like too many categories and we should rather provide easy ways to filter?