I have a stupid question but I'm super confused from it. If React Native has Async Storage available, why should one use Redux and Redux Thunk? Why can't we just save all the data in the Async Storage? What is the purpose/benefit of using Redux over Async Storage?
They are different things and serve different purposes.
Async Storage is a simple key/value store. It only works with strings. So you can do, AsyncStorage.set("someKey", "someValue") That's all it does. It's purpose is to save/persist data on the file system of the phone so that it can be used over multiple app sessions (closing and opening the app)
Redux is a full state management solution that allows you to keep any kind of javascript data in memory during the running of the app and have it available anywhere in the app. Think about how in a react-native app two different components cannot easily see and modify each others state, there has to be some way for them to communicate. Basically, Redux helps you do this.
Considering your level of understanding, I would recommend looking into React Context before you tackle Redux. It is a significantly simpler state management solution than Redux. It is included with React/doesn't require a different code structure and will also help you understand the purpose of Redux.