Gatsby GraphQL Data Layer Mastery Hub: The Industry Foundati

Within the Gatsby GraphQL data layer, what is the primary mechanism for defining the schema and its types, ensuring data consistency and enabling powerful querying?

When encountering a scenario where a single Markdown file needs to be parsed and transformed into multiple distinct GraphQL nodes (e.g., one for the main content, another for a table of contents), which Gatsby node API is most appropriately leveraged for this granular data structuring?

Consider a situation where you need to enforce a strict relationship between two different data types within your Gatsby GraphQL schema, such as ensuring every "Product" node has a valid "Category" node associated with it. Which Gatsby GraphQL data layer feature is best suited for declaring and validating these relationships?