A GraphQL & Relay developer designs and implements APIs using GraphQL, a data query language for APIs. They use Relay, a JavaScript framework, for effective client-server communication, optimizing data fetching process to improve application performance. They design schemas, resolve queries, build connections between existing databases and GraphQL servers, and troubleshoot any issues. They also ensure efficient data loading from the server to the client, reducing network traffic. The developer works closely with front-end teams to align data requirements and provides maintenance and continuous optimization of the API. They also ensure the system's security and data protection.