They are. If you make a react native project using the official boilerplate generator, the resulting project will be able to run on both Android and iOS out of the box, with no modifications. It's true that you might need to use native APIs and components for specific features, but the everyday UI building blocks of RN are cross platform. These are(in my mind): the flexbox layout engine, styleable box/image/text components, animation system, and a touch and gesture interaction system. In addition many of the other parts are cross platform too: geolocation, device dimensions, activity indicators, alerts etc.
Even some of the built-in components are platform specific though, such as the segmented control for iOS and view pager for Android. But the nice thing is you don't need to create a separate project to use these, you can just chose the appropriate one at runtime based on the platform.
Even some of the built-in components are platform specific though, such as the segmented control for iOS and view pager for Android. But the nice thing is you don't need to create a separate project to use these, you can just chose the appropriate one at runtime based on the platform.