As far as I could tell, there is no passive adapter that converts Displayport to usb-c, you can only go the other way. The only way to use a non usb-c device is to use a capture card to capture the HDMI/Displayport signal and retransmit it over usb.
There are two ways to carry video signals over type-c: DisplayPort alt mode, and Thunderbolt. DP Alt mode can work in either direction with a passive adapter or cable: a monitor with a Type-C DP input can be driven by a graphics card with a regular DP output connector, because it's all DisplayPort signalling either way.
Thunderbolt encapsulates the DisplayPort data in Thunderbolt packets, so both endpoints of the link need to be full-featured Thunderbolt devices.
