What do you need to master to be considered to be a React Native Expert? To gain precious knowledge, you should start with the following:

Start With React Native

The architecture of React-Native Apps. It should teach you about Core components of React Native (View, ListView, Image, ScrollView, Dimensions, Text, Touchable, Modal, WebView, Vibration, StatusBar) and their hierarchy. You should know RN Tools e.g. like what is metro/packager, RN bridge, watchman, npm.

With writing code, you may need to debug your app, using either default Chrome Developer Tools or by accessing system logs (either with Logcat or XCode console).

You will also need to know how permissions e.g. for camera work in Android and iOS.

A great book should explain why you may need data-management libraries (like Redux or Mobx), their React bindings, how to use them properly and explain when you would not need any of them.

You should learn about handling side-effects (e.g. with Redux Saga or Redux-Thunk). If you need a database, you should have a brief overview of a few possibilities just to identify which one fits your needs (e.g. Realm, SQLite, Firebase, etc.).

You should gain brief knowledge about navigation libraries (React Navigation) and navigation-core components (e.g. StackNavigator, DrawerNavigator, Tabs-like navigation, Switch Navigators).

And last but not least, you should read a lot about styling in RN (also here) and the Flexbox because the design is the thing that sells in 2020.

Continue With Native Platform Development side

For native-platform oriented projects, you may need to know how to write RN custom modules (platform-dependent code) using different coding-styles (callbacks, promises or event- emitting). With custom modules, you should know how to use branching to separate platform-specific code into “.ios” / “.android” files.

If you are targeting more specific platform dependent solutions, you will need to learn also native-development languages like Objective C, Swift, Kotlin or Java.

Learn how UX and UI work

To be an expert in the mobile development world, knowledge of basic principles of UX and UI is a must. And how do you gain this knowledge?

Read this:

UX Design for Mobile
Get proficient in building beautiful and appealing mobile interfaces (UI) with this complete mobile user experience (UX) design guide.