OneSignal's push notification API is designed to deliver customizable notifications across various devices and platforms, making it an essential tool for developers creating web or mobile applications. The API is free to use with no limits on calls, notifications, or devices, and it provides access to analytics for optimizing notification performance through A/B testing. While primarily used for sending notifications to users, developers also utilize the API to streamline workflows by integrating it with other services to trigger notifications for specific actions, such as publishing a blog post or creating a pull request. The API, known for its scalability, handles over 400,000 requests per minute and serves more than a billion devices, leveraging a stack that includes Ruby, Rails, and PostgreSQL. OneSignal monetizes by offering custom solutions to enterprise clients and by selling aggregated data, demonstrating its capability to handle high volumes and meet diverse developer needs.