Well I wrote not React Native for mobile dev, but I'm the wrong person to ask in any case. I learn best by starting a project with something I'm totally unfamiliar with and just... going. In fact I find learning other ways almost intolerably dull and am unable to stay focused, when it comes to computer stuff (other topics, I don't have that problem). So from my perspective, the answer to "how best do I learn this computer thing?" is going to be "just start a project—ideally one that pays" about 95% of the time. Or pick a simple-enough-looking but not-dead project in the tech on GitHub and start contributing.
I gather other people like reading docs (OK, that's alright) or tech books (ugh) or watching videos (double ugh) or listening to podcasts (triple ugh) to learn computer things. I almost never do most of that (aside from consulting docs), especially not when first approaching something. I am, therefore, just about the worst person to ask about resources for learning any computer thing.
My only suggestions other than that would be to use Create React App, because the alternative is the sheer cosmic horror or a complex JavaScript build, and that goes at least double if you're doing React Native rather than browser-targeted React, and to start a React project with TypeScript because without it JavaScript is kinda miserable, and there's little reason not to.
