I don't know, in theory that's why they get 30%, to figure that out.
So what platform has 4.5 million products and can showcase them all?
They most certainly do. If you've ever worked in retail you'd be familiar with the daily restocking sheet, where we had to move certain products to the eye level shelf and other ones down.
So some products get showcased and some languish in obscurity. Apple also showcases a few apps....
And those products that get showcased on the end cap pay extra. You can also (sadly) pay extra to run search based ads in the App Store.