> USB-C HDMI adapters are all DisplayPort adapters that additionally tunnel DVI traffic
Not tunnel, convert: these are active adapters that take in a DisplayPort signal from the computer and re-encode it as a DVI/HDMI output signal. The computer doesn't need to know or care that there's HDMI happening downstream, because it's still just outputting DisplayPort signal. It's not like Thunderbolt where DisplayPort traffic is tunneled inside a different signalling mode that the host machine supports.
This is why USB-C to HDMI adapters are unidirectional (at least, all the mainstream commodity ones). You'd need a different converter chip to do the conversion in the other direction.
Not tunnel, convert: these are active adapters that take in a DisplayPort signal from the computer and re-encode it as a DVI/HDMI output signal. The computer doesn't need to know or care that there's HDMI happening downstream, because it's still just outputting DisplayPort signal. It's not like Thunderbolt where DisplayPort traffic is tunneled inside a different signalling mode that the host machine supports.
This is why USB-C to HDMI adapters are unidirectional (at least, all the mainstream commodity ones). You'd need a different converter chip to do the conversion in the other direction.