System RAM is not directly related to Video RAM. Your CPU, MoBo, socket, chipset, etc are made to have DDR4 RAM modules plugged, the system RAM, and the graphics card has its own Video RAM built-in which would be appropriate for the bandwith and space needed for its rendering capabilities and be accesible only for the GPU, so there isn't any incompatiblity here.
Medium and high end graphics cards use GDDR5/X/6 or HBM/2 memories which again are totally different to the usual DDR3/4 for the system, attending to higher speed and latencies needed.