The difference between a tool customers use directly (product) and a layer other businesses build their own products on top of, where you take a cut while they do the work (platform).
Shopify doesn't list inventory and doesn't ship boxes. It runs the storefront software, the payments rails, and a developer API that thousands of third-party apps plug into. As of 2024 the platform powered roughly 4.6 million live stores and over $290 billion in annual gross merchandise value, none of which Shopify owned. The economics work because the merchants do the selling.