React Native

In 2015 Facebook released a platform to allow for writing mobile apps using react like components but that would be converted into actual native elements in the app. The code that is reused between the apps is JS or TS code so familiarity with JS and react will give you enough of a foundation to start working on RN projects.

This is a great initiative to get developers from the web onto mobile platforms and allow companies to approach product development in a leaner manner, making developer skills more transferrable between the seemingly distinct environments.

The ecosystem of React Native tooling has refined a lot since 2015 and the trend is to have similar cli tools like create-react-app and very similar naming conventions as well, so when coming from React the environment is very intuitive.

The docs are a great place to get started and can give you a very good understanding of how to write a basic react-native app, however like with all the react libraries the aim is to be very lighweight so as not to have any unrequested extras. While this adds very little cognitive overload in this particular case it puts the developer at a huge disadvantage because the ecosystem is very complex and the more automation you can have out of the box the better off you are.