The gpu that is connected to the monitor that is set to primary in windows will determine which gpu is the primary. So if the monitor on the gpu is primary, the gpu will do the graphics processing in the case of games, 3d rendering, any gpu acceleration unless the software let's you choose, etc. But the igpu is outputting to the monitor connected to it. Although I have seen some software override when the igpu is set to primary since it can detect the gpu.