All you see at a pixel is an estimation of the reflected light from all the light sources from the surfaces that provide a potential light path from the camera

All you see at a pixel is an estimation of the reflected light from all the light sources from the surfaces that provide a potential light path from the camera to the source (bounced accordingly off intermediate surfaces.) In practice, it’s just a ton of hacks to estimate this value. It turns out if you do this reasonably accurately across all pixels and surfaces you get a pretty picture. Anything else is just optimization. Read PBRT to learn how basic irradiance estimation should work and then rasterization becomes less about understanding the goal and more about understanding the hacks needed to achieve the goal in real time.