And the products on the shelves of stores are not the store's products.

Apple sells apps. Apps that were made by others. Walmart sells X. X that was made by others.

Apple didn't buy the apps from the developers, they provide a marketplace. Don't most supermarkets own their inventory?

I think a lot of businesses "borrow" the inventory from the supplier and only pay them once the inventory is sold. It's not the same as a marketplace, per-se but considering you pay apple, download from apple servers, after apple does "QA" and follows apple rules, i'd say its pretty similar to a store that sources select products and provides the checkout infrastructure and distribution logistics.