A Party APIs developer integrates external APIs into applications to add functionality or access services. They read and understand API documentation, write code to call the API, handle responses, and troubleshoot issues. They often use REST or SOAP protocols and may need to work with JSON or XML data formats. They ensure security through API keys or OAuth. Their work enables applications to interact with external systems such as social media platforms, payment gateways, mapping services, or data providers, enhancing the app's capabilities without developing those features from scratch, thus saving time and resources.