At the moment Appearance > Templates only lists templates that have been created from scratch, or theme templates that have been customised by a user at the site.

It seems sensible to display all templates here, including those supplied by themes and plugins. Just like we do did in the Site Editor. This provides a convenient pathway for folks to go and edit all the templates in use at their site.

Since templates supplied by third parties cannot be edited, we may need to disable bulk actions for those templates.