I have a question about skins/containers and portals. If I install a skin or container in the portals/_default folder, should I be able to see/use that in other portals (child or parents)? Or do I need to install them in the specific portal?

thank you!

If a portal uses a skin installed in _default folder (skins installed under Host rights) can be shared by other portals. This leads to all sorts of issues if you decide to modify something in a skin for a particular portal. Other than that, it is largely irrelevant for development. Of course, there are many business model considerations to take into account, but from programming perspective this seems to be the most relevant one.