React Native is not converting Javascript to Objective-C/Java.
React Native is not using WebView to render the page like Cordova. React Native uses JavascriptCore which is the JavaScript engine that powers Safari. That is the reason you can't write HTML/CSS inside React Native.
If you want to access the Hardware layers, You can use plugins like Cordova. But, React Native doesn't have many plugins like Cordova. React Native community is growing day by day. But, React Native have plugins for all the essential features like Fingerprint, GPS, Camera, etc., So, Java/Objective-C knowledge is not mandatory. But, if you have Java/Objective-C knowledge, It will be helpful. When I started making React Native, I had a requirement to use Android's Shared Preference. There was no plugin for accessing Android's Shared Preference from React Native. So, My friend gave me the snippet for using Shared Preferences, Then I created React Native module for it and published as a plugin. Now, Many people are using my plugin. Like that, All the Android's and iOS's features will be covered soon.