A testing developer is responsible for designing, implementing, and executing tests to ensure the functionality and quality of software. They use various types of testing methods such as unit testing, integration testing, system testing, and regression testing. Unit testing checks individual components, while integration testing verifies the interaction between different components. System testing ensures the system works as a whole, and regression testing checks for any new bugs after changes have been made. They also use performance testing to measure speed, robustness, and stability. Ultimately, they identify and fix bugs, ensuring the software meets the desired requirements and standards.