@Pogogunner At least on Debian based distros, it’s all part of the driver installation.
As for how it works at the hardware/kernel level the iGPU take some of system RAM to use as VRAM, so all the kernel has to do is give the dGPU a DMA buffer into that. The final piece is for the iGPU driver to send a synchronisation signal to the dGPU when it’s ready to receive the (partial-)frame.
@j4k3 Filtering out everything about them is the simplest way to block anything critical of them.