For b.) you can install the app “Team folders” (earlier Group folders). Then you can create in Administration / Team folders (https://cloud.server.tld/settings/admin/groupfolders) a Team folder (earlier Group folders) and allow a user, group or a team to manage the Team folder.
It was announced in a way, I expected it to be a core function of the product, not an extension (through apps). But fair enough, there is the Team folders app and it was this, what I was looking for.
The advantage of Nextcloud is the possibility to do without certain functions. Apps can be partially deactivated. Other apps do not need to be installed at all.
The Teams app ( formerly known as Circles) for managing teams can be deactivated. It cannot be uninstalled.
The Teams folders ( formerly known as Group folders) app does not need to be installed.
This is particularly useful for single-user systems where individuals want to manage their data without teams.