I made an android app for a food delivery company that never came to fruition. The app uses firebase to communicate between the client app, restaurant app, courier app, and the realtime database itself. It allows users to order food, then alerts the restaurant, when the food is ready, it will update the customer, and send their location to a courier who delivers the food. Basically its the same as any delivery app. This was my first complex android app, and I was learning android studio as I went. If I were to do it again, there are things I would change, however given my lack of experience, I would say it turned out surprisingly well.