Keep in mind that this is NOT (primarily) about cross platform application development. As the developers say, "Learn once, write anywhere".
It's about building native applications in the React style (declarative, componentised), with very rapid turnaround because much of your logic and layout is defined in JavaScript. Instead of recompiling your app's native code all the time, you're just reloading JavaScript within it.
It's possible that some components could be shared between platforms, but ultimately I think most of them will be different, reflecting the UI structure of the target environment. There's probably more scope for sharing higher level non-UI-element components.
It's about building native applications in the React style (declarative, componentised), with very rapid turnaround because much of your logic and layout is defined in JavaScript. Instead of recompiling your app's native code all the time, you're just reloading JavaScript within it.
It's possible that some components could be shared between platforms, but ultimately I think most of them will be different, reflecting the UI structure of the target environment. There's probably more scope for sharing higher level non-UI-element components.