YouTube is one of the most popular mobile apps for both iOS and Android. Apple's App Store considers it an "Essential" app and it rests comfortably within the top 10 free apps at almost all times. Changing it is a big deal and a tricky process, involving many phases and the coordination of hundreds of employees.
My first project after joining YouTube was to work on a complete redesign of the app, which would change the overall navigational structure and update the UI to Google's Material Design specifications. My role was a prototyper, working on key interactions and user-testing them in many different situations.