he had already worked out just about every single detail of the painting with the use of many pencil sketches and colour studies. He would have then created a final pencil draft with all of the details in place (except, perhaps for mechanical details such as textures of cloth. But even then, he would have worked these details out in his head at this point) He would then transfer this drawing onto the large canvas, up-scaling it in the process. Then, and only then would he begin to apply the paint
What you described there completely qualifies as a "highly iterative process", which was my original point. The medium of iteration is largely irrelevant.