Not sure where I said that but to clear up the confusion, the motherboard would output video over usb c port if you use a usb-c to displayport cable or a usb-c to display port dongle. Depending on the motherboard you might be forced to choose between hdmi/usb c video. Read the manual for correct info on how the motherboard behaves.
As I said, that splitter is doing software vodoo to allow that feature. Nothing to do with usb a port as such. Technologies like DisplayLink exist which are not dependent on hardware functionalities. Not sure if startech adapter is also using displaylink
As I said, depends on the motherboard. Both B450 and Z490 are now 4-5 year old chipsets.
