In a headless setup, Shopify APIs connect the frontend with the backend by securely exchanging product, customer, cart, and checkout data. The storefront sends requests to Shopify, which processes them and returns the required information in real time. This approach allows developers to create fast, flexible, and highly customized shopping experiences while keeping Shopify as the commerce engine. Shopify Hydrogen further simplifies headless development by providing optimized tools, components, and frameworks for building modern storefronts efficiently.