This has become sort of spread out across a bunch of posts. Let me summarize, (I realize that probably a lot of people already know this... but for anyone trying to figure it all out):
Duet 2 can be on a WiFi or Ethernet network (depending on hardware model). Duet 3 can be on WiFi or Ethernet or anything else supported by the ribbon-cable-attached Single Board Computer (i.e. Pi) that is part of the Duet 3 architecture. (It is also possible to run a D3 'stand alone', meaning with no attached SBC. I'm going to ignore that for right now).
Once your Duet 2 or 3 is on a network, then anything that can run a browser can present DWC (the Duet Web Control) interface. This could be a PC, a Tablet, a Phone, etc, etc. This can be on a desktop, or attached to the frame of the printer, or wherever. With appropriate security and proxy setup, this could transit the internet and be anywhere on the planet.
A PanelDue can be plugged into a Duet 2 or 3; this is a "TTL Serial" connection and does not use any form of WiFi or Ethernet. As such, it can run on a few meters of cable, but not hundreds of meters. The PanelDue itself can have any of several sizes of touchscreen. The UI on a PanelDue is designed to operate the printer, and has several limitations as compared to 'full' DWC.
All above is the same (or very close) for D2 or D3.
A Duet 3 with attached Single Board Computer (that is, a Pi) offers one more option: The Pi will be running DWC on its HDMI terminal session. Therefore, any combination of mouse/pad, keyboard, display will present a 'full' DWC interface. This includes any of the many touchscreens already made for Pi that emulate mouse/keyboard via touch. Given that the Pi must be ribbon cabled to the Duet 3, and that the Touchscreen must be HDMI cabled + USB (typically) to the Pi, it makes most sense for this setup to be physically on the printer.
As long as the above is, it is still very summarized. Hopefully it is reasonably accurate; corrections are certainly welcome.