The link below is to a video that explains quite well, with illustrations, how realistic and responsive 3D scenery and objects are generated. Yes, the bulk of the work performed today is done by 3rd party gaming engines like the Unreal Engine and others. So from a developer point of view, they don’t really have to get their hands dirty any more with the nitty-gritty mathematics.
But if you consider the number and complexity of calculations made for each pixel (as the video explains) and then multiply that by the number of pixels on the screen, and the number of screen refreshes every second, it becomes quite mind-blowing. It is no wonder that GPUs are so powerful and cost more than the rest of the PC all combined.