Is Swift native?
Swift, being the iOS-native language designed by Apple to power apps has obviously no issues with integrating into the iOS design and leveraging all the UX principles. On the other hand, building the app with Swift is, in fact, coding a native app from scratch.
Can I use Swift With React Native?
For iOS, you can choose between Swift and Objective-C to implement these features on your app. Mostly React Native code is written with Objective-C because React Native core uses Yoga lib for layout that works better bridging with Objective-C because is written in C++.
Should you use React Native or Swift?
Swift is a clear and concise programming language. That's why it suits large enterprise apps more than React Native. Also, building an iOS app with Swift is faster than with React Native. Still, you'll need more time and money if you plan to make a native Android app as well.