The User Shares are in /mnt/user. If you have a User Share named Music, it is at /mnt/user/Music. There are also the disk paths, /mnt/disk1, /mnt/disk2, ..., /mnt/cache, and possibly others if you have other "pools" in 6.9+. And there are the /mnt/disks and /mnt/remotes created by Unassigned Devices.
The User Shares are simply the aggregate of all top level folders on the array and pools. A folder named for the share is created at the top level of the disks as needed according to the settings for the share. Conversely, any folder at the top level is automatically a share named for the folder, whether or not you created it as a User Share in the webUI. Any share you haven't made settings for will have default settings.
Since the user shares are just another view of the disks, you must not mix disks and user shares when moving / copying, because Linux doesn't know that the source and the destination might be the same file and it will try to overwrite what it is trying to read.