A Butterknife Library developer designs and implements Android applications using the Butterknife Library. This library simplifies the process of binding views and callbacks to fields and methods, reducing boilerplate code and making the code cleaner and easier to read. The developer uses annotations provided by the library to connect views and callbacks, improving efficiency and reducing bugs. They also maintain and update the applications, ensuring compatibility with different Android versions and devices. They collaborate with other team members to optimize user interface and user experience, and debug any issues that may arise during development or after deployment.