In terms of how much React/JavaScript code you should expect to write vs. how much Native code for a production app, it obviously depends a lot on the app but maybe 80/20 might be a reasonable (if totally arbitrary) expectation.

Here's a good high-level talk which touches on the balance somewhat: https://www.youtube.com/watch?v=abSNo2P9mMM.

You should only expect to write pure native code when you hit a performance barrier with some particular custom component you're creating and there isn't an existing JS api or a standard/3rd party component which does everything you need already. But when you do hit this barrier, wiring up your own natively implemented module is pretty straightforward.