DeX works when streaming to Samsung Smart TV and also works when connecting monitor directly to monitor with HDMI to USB-C cable, but does not work when Phone directly connected to PC via USB cable.
With Samsung DeX for PC application installed on Windows 10 Professional PC and the Note 20 Ultra is connected to the PC via USB-C cable, the PC recognizes the Note 20 device and opens Windows File Explorer application to view files on the Note 20 Phone file structure and also the micro SD card file structure installed in the Note 20 Phone. The PC also opens the Samsung DeX for PC application, provides a status of "Samsung DeX is getting read to start. Tap Start now on your phone." However, the phone does not start DeX and does not provide a screen with any place to tap Start. When swiping down on the home screen to access the Quick Panel on the Phone and then tapping on the DeX icon, the DeX panel that opens shows an information panel at the top, which can be swept to the right to show the 3 methods of usnig DeX (stream to TV, connect cable directly to monitor, connect to PC), but there are no controls with any of these displays. Below this is a panel to allow selecting the TV to stream to. It either shows my Smart Samsung TV if it is on or shows no devices if my TV is off. There are no other controls available. Selecting the 3 vertical dots in the upper right hand corner leads to the DeX Off status and the ability to turn it on, which takes the screen back to the same DeX status screen with the only option to connect to a TV.
So the PC is recognizing the USB connected phone, but something on the phone is not recognizing that the PC is ready to start a DeX for PC session or something has the phone DeX stuck in the streaming to TV mode when connected with USB cable directly to the PC.