No, you can configure it to just build and launch for iOS or Android separately. But we do recommend iterating on both in parallel for most of the UI work, just to make sure that everything stays in sync.

For framework/library development, you can of course build and test separately for each platform.

And if we're talking Expo, that's only for prebuilds of course; once you've got the native app installed then you can absolutely code and see updates in near real-time on both Android and iOS devices.