There are a number of different ways to do it. I have a couple of modern TV's for example, that have HDMI inputs where there is actually little to no video data sent over the link. Instead, the TV actually includes the GPU, and all that's used on the HDMI connection is a high speed serial bit stream of GPU commands. Like wise I also have USB3 to HDMI/DVI/VGA adapters that basically have the same GPU inside the adapter, and take a serial bit stream from the USB3 port, in much the same way my TV takes it's GPU input over the HDMI serial link. Broadcom make a LOT of SOC's that can do this decoding in real time, and many of the devices I open and investigate (esp the older ones) tend to have broadcom chips in them. In some cases though, yes it is just a straight through high speed serial link, USB3 to HDMI, you tend to be able to spot these ones though, they are generally smaller than the GPU based ones, and they won't work with some makes of monitors (EG: ones that don't have their own on-board GPU)
