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Some USB-C adapters offer a video output as well as a standard USB output. Such an adapter (for example with a USB port and a HDMI port) can handle simultaneous outputs: i can have video coming from the HDMI port while transfering data to a USB disk with the same adapter.
I am assuming the video output is coming from an USB alternate mode (HDMI in our example), so this means both signals go through the same pin. How does this work in practice?
This mode is actually called DP-ALT mode, DisplayPort Alternate mode. USB 3.0/1/2 cable has four high-speed differential pairs. In normal USB operations only two pairs are used, the other two are just hanging unused. In DP-ALT mode the host system has a mux that connects two DP lanes into two unused pairs on USB Type-C connector. More details can be found in this early article from anandtech, and here is an illustration of the idea how it is usually implemented:
There are several modes for DP+USB, with 2-lane DP variant, and also a 4-lane DP with USB over AUX lines, or USB-2.0 only over regular D+/D- wires.
To get into this mode, both provider and consumer must negotioate over CC lines using PD messaging system.
