A highly scalable systems developer designs and implements software systems that can handle significant growth in users, traffic, or data size without loss of performance. They plan for future growth, ensuring systems can efficiently manage increased demand. They optimize data storage, manage network resources, and modify applications for scalability. They often work with cloud technologies, distributed databases, and big data processing frameworks. They also monitor system performance, troubleshoot issues, and regularly update the system's infrastructure. Their goal is to ensure the system remains functional and efficient, even as it handles vast amounts of information or users.