Apple Music and iCloud storage

The reserved total cloud space for your family group is shared, but the content in the cloud stays individual, unless members choose to share certain content, and mark/move it as such. Family Sharing - Share an iCloud storage plan with your family - Apple Support

By default, B can’t see D’s backups/photos/videos/documents/messages/mail/memos/etc.

Yes. iCloud Storage plans can be turned off separately for Family Sharing.

if possible, can I only share the iCloud storage to some members?

Looks like no.

See >>> https://support.apple.com/en-us/HT208147