If you want to see how a rasterizer (the thing that displays polygons on the screen) is done in software, follow Eric's suggestion and check out mesa3d or google "rasterizer".
If you want to see how a rasterizer is done on hardware, I'm afraid the only thing you can have access to is a photo of a GPU die.
If you want to see how a 3D engine is done, take a look at Ogre, Irrlicht or any other.
If you want to know what happens under the hood when you call glDrawArray on a geForce, you can't. The drivers are proprietary. They more or less forward the call to the graphic card, but it's such a simplification... for instance, a typical DirectX call takes more than 10000 cpu instructions.
