Which Shopify APIs are used for a headless Shopify build?

When working with a headless Shopify store, it's primarily the Storefront APIs that will be used. The Storefront API uses the GraphQL format, which certainly helps with consistency and flexibility.

The Storefront API contains a vast number of possible queries, which will be mostly suitable alone for even a fairly complex headless implementation.

Shopify also offers the Admin API – which we frequently use for custom apps or extended/more complex backend operations. The new Admin API also uses GraphQL.

Besides Shopify's API library, another little-known feature of Shopify is Webhook notifications. These are gold and essentially facilitate the development of code-based "flows". With webhooks, we're able to take events upon certain actions that may take place within the Shopify store, and build out a code-driven reaction/response to those events in an external app. As part of complex headless builds, we'll typically leverage webhooks as part of the implementation so that further actions can be conducted upon certain events being trigged within Shopify.

To note: Shopify's legacy REST APIs are no longer supported, so should not be used in new development projects. Older projects utilizing the REST APIs should, ideally, transition to GraphQL as soon as possible.