I added a new user USER11. This user needs space on our cloud but must not see more than his own file. At the moment USER11 can save files on the cloud, but they are not part of our nextcloud files structure as it is not searchable, viewable, and accessible from admin users. How can I solve that, that the USER1 files are part of our nextcloud files structure, but USER1 can see only his files?
Either you can share a folder with this user, so he can change things there (and you can give access to other users as well). Or use the groupfolder app. Apart from this shared storage, the native Nextcloud storage is considered to be the user’s storage where it is up to the user what to share.
Doing more refined permission settings that are different for subfolder etc., for such more complicated things, it can be worth using external storage and the permission capabilities of these external storages (e.g. NFS, SMB, …). Then you can authenticate each user individually with his credentials. You can do this even as primary external storage. However this is a bit more tricky to set up and is not documented very well (I didn’t check recently).
Perhaps you like the Guest App. (video)