And if you plug a Samsung Galaxy S9/10/Note into that same USB-C, a full desktop will appear on your monitor as well using your existing ethernet, mouse, keyboard and audio outputs. Something not possible with full sized HDMI ports.
Linux on Dex was never particularly useful.
Is there some setting I need to enable on my phone, or is my setup just different in some way?
There's also a setting to switch between modes under Settings -> Connections -> More connection settings -> HDMI mode.