The only people who are positioned to provide a definitive answer are executives at said companies. As noted, large OEMs buy components in bulk and pay less per part than a builder, and they also get Windows licenses in bulk. Then there's a split in the difference between the bill of materials including a Windows license and the sale cost: that difference gets divided somehow between the store and the OEM PC maker. Then, the question of how much the OEM revenues become profit is determined by their internal costs: labor, infrastructure, and others.