"Learn once, write everywhere" is what they claim... and while it's not wrong, it is to a certain extend... just a claim. Please don't get me wrong, I am deeply indepted to both React and React Native, as they allow me to create applications that would me much more difficult and timeconsuming to create without these tools. And I have really been enjoying to work with them over the last ~ 5 years.
The mobile platform and a webbrowser are, however, fundamentally different at the end of the day, and if you want to accomplish anything substantial in either environment, you will have to learn about their respective particularities. And this means to learn at least... twice.