I've always wondered this. What happens if you plug two monitors in the PC, one in the motherboard and one in the GPU, and then opened a game and dragged it across the monitors? Well it turns out it just uses whatever GPU you first opened the game on.
In the video below, I opened Fornite (cringe, i know) on the left monitor which was hooked up to the RX 570 and then dragged it to the right monitor which was plugged into the iGPU and it kept running at the same framerate. I then unplugged the monitor from the GPU and even though the remaining monitor was plugged into the motherboard, it kept running at the same framerate.
Linus made a video where he used Mining-made GPU's for gaming by using a little trick to feed the GPU's output to the motherboard, but I guess this is just built into- and used by Windows by default?
If so, does anybody know how this could be manually turned on? This would be really useful for someone who has a GPU that works but just can't output an image on its own ports.