"net use" does not create drives in Explorer on Windows 10

C:\Windows\system32>net use  
OK           X:        \\192.168.8.109\Public    Microsoft Windows Network  
OK           Y:        \\192.168.8.109\DATA      Microsoft Windows Network  
OK           Z:        \\192.168.8.109\MEDIA     Microsoft Windows Network 

Command-line instructions like dir x: work as expected.

In Windows Explorer, why do these 3 mapped drives not show up? Suggestions and diagnostic questions are appreciated.

Mapped network drives created by an elevated process will not be visible to an unelevated process and vice versa. This has been the case since Windows Vista. Since Explorer doesn't run elevated (unless you fully disable UAC), it doesn't see the drives you made with your administrative command prompt.

To disable that isolation, you can modify the Registry, as instructed by this TechNet article. Navigate to this key:

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System

Create a DWORD entry named EnableLinkedConnections with a value of 1. Reboot the computer. Network drives mapped by the same user, even under different security contexts, will always appear to programs running as that user.