Shared Drive folders in GSuite are backed up from the folder owner.
If a folder is shared with another user and the user created a sub-folder there but didn't share it with the owner of the root folder, then that sub-folder will not be backed up in the root folder owner's account until the sub-folder is shared with the root folder owner.
If the user also has Shield Backup GSuite in use, the folders shared with him/her are not backed up unless moved from the "Shared with me" section to their own Drive. Once moved, all of them will be backed up.