Photorealistic rendering means that we put virtual objects in a scene, assign material models to them, and then run a light simulation program to create a beautiful image.
Make sure to have a look at the paper, there are many more experiments that went similarly, including a case where the AI found a bug in a physics simulation program to get an edge.
It is typically done by using light simulation programs where we use the laws of physics to simulate the path of millions and millions of light rays as they bounce off of different objects in a scene.