PostgreSQL has a powerful introspection API which allows us to ask it all about a database - what schemas, tables, columns, constraints, indexes, functions, comments, etc it has - and we use this information to construct the GraphQL schema automatically. You provide the list of schemas to inspect via the pgSchemas setting, e.g. pgSchemas: ["public"].

If you're interested to see how we do this, the introspection query can be found in our GitHub.