The Tina GraphQL Gateway builds its schema from your site’s content model. This is defined by
This schema file is stored within a .tina directory in your repo. When you create or edit one of the front-matter template files or your settings file Tina will regenerate your schema based on your content types. The gql builder service is responsible for building out this entire GraphQL schema for a given .tina config.
At the top of the schema is a document query, this query returns the document, which can be one of any number of templates defined in the .tina config. From there, each field in the given template is used to build out the rest of the schema, so each template field is built by the type in it's definition.
To learn more, visit GraphQL Gateway API