Interesting.
Also note that, while File Explorer and Windows file/open dialog boxes don't accept "\\?"-style paths at all, you can use the "Run" command (Windows Key+R or choose "Run" from the Start Menu) to open any available (connected and online) volume in File Explorer using a path of the form "\\?\Volume{UUID}\"[1], even if it hasn't been assigned a drive letter or mount point.
Note that, if UAC is enabled, this won't work for EFI system partitions unless you're running Explorer elevated, which can be tricky. You used to be able to kill the explorer.exe process and start a new one from an elevated command prompt or Task Manager, but this no longer works in Windows 11 (Explorer starts, but not elevated).
[1] For GPT disks, this UUID is the GPT partition's unique ID.
I have no idea how the ID is calculated for non-GPT disks, though I have noticed a few interesting properties:
• For volumes on MBR-partitioned disks, all but the first eight bytes of the UUID are zero.
• The UUID is not obviously related to a FAT filesystem's serial number.
• UUIDs are assigned to devices with removable media like optical drives and SD card readers even when empty, and do not change when media is inserted or removed.
In any case, the easiest way I know of find this is using PowerShell: use "Get-Disk" to find the disk number (and if this doesn't give you enough information to identify the correct disk, use "Get-Disk | Format-List" for more information), "Get-Partition -DiskNumber n" to find the partition number (if the disk is partitioned), and, finally, "Get-Partition -DiskNumber n -PartitionNumber m | Get-Volume | select Path" to get the desired path.
In the case of non-hard disk devices like optical drives, "Get-Disk" and/or "Get-Partition" may not be available, so use "Get-Disk | Format-List" for detailed information on all available volumes, including all volume paths and, hopefully, enough additional information to identify the volume you want.
You can also find volume paths with WinObj or the fsutil command, but PowerShell provides more useful information to help identify the correct volume.