An API usage developer designs, implements, and manages software applications using Application Programming Interfaces (APIs). They use APIs to enable communication between different software systems, enhancing functionality and interoperability. Their role involves understanding API capabilities, integrating them into software applications, and troubleshooting any issues. They also monitor API performance and ensure security measures are in place. Additionally, they may be involved in designing and developing new APIs, documenting API usage for other developers, and constantly updating their knowledge to stay abreast with the latest API technologies and best practices.