A React Native app is a compiled app that is running some Javascript. Whenever you build and run your React Native project, a packager starts up called Metro. You’ve probably seen this output in your terminal before, letting your know the packager is running.
The packager does a few things:
Combines all your Javascript code into a single file, and translates any Javascript code that your device won’t understand (like JSX or some of the newer JS syntax).
Converts assets (e.g. PNG files) into objects that can be displayed by an Image component.
reference:
https://hackernoon.com/understanding-expo-for-react-native-7bf23054bbcd