The GUI layer in a static sense. You can't have native realtime interactions with the GUI, for instance dragging and interactive animations.

You can choose one of two things:

1) Run a single precomputed action or animation in native-land when receiving an event from JS. No interaction is possible.

2) Run a requestAnimationFrame loop in JS which is passing the bridge (6 API calls on Android - 3 layers per direction) 60-120 times per second in async mode. Enjoy the jank.

React Native isn't afraid to expose platform specific details when abstracting away cross platform APIs in uniform APIs. In my experience, it was fairly easy to use a native UI library inside a React Native app. (It takes more work to embed a React Native view inside a native view that's embedded in a react native view, but it's possible.)

I've seen both uses of the term "native"; it's an overloaded term. Some use it to mean "native UI", and others use it to mean "natively compiled". I meant it in the latter case. I actually prefer wholly custom UI, as most apps do these days that are cross-platform, because it allows for very interesting user experiences. Of course, some people disagree, wanting everything to be native-looking and feeling to their platform, such as Linux apps all using GTK or macOS all using Cocoa.