Before we get into talking about light estimation for AR, let's step back and review the rendering process of a 3D model. Take a look at the following diagram that explains the rendering process at a high level:
Now, the diagram only visually demonstrates the rendering process. Geometry and vertex shaders never actually render a wireframe model. Rather, they only position and color vertices and surfaces, which are then fed into the pixel/fragment and lighting shaders. This last step is called rasterization and represents the final step when the 2D image is generated or rasterized.