For that I am using the compiler which allows C#, Go, Swift code to mixed. It’s really great. So with some work you could use existing C# code even in your mobile apps. I am liking it so far :)

Which compiler is this?

I think this is a reference to RemObjects Elements

Swift will also work on Windows, and of course on MacOS (and Linux). If you prefer, you could use Kotlin in a similar role via kotlin-native.

But Swift won't work on Android. And Kotlin Native is severely compromised.

The lingua franca options are effectively Xamarin, Flutter, and React Native. Xamarin's dev experience sucks and Flutter's perf isn't much different from React Native with a way less mature ecosystem.

There are people who should write platform-native mobile apps. For everybody else, React Native is fine. And, for my money, TypeScript is the most right programming language easily available on major platforms today, so that makes this one pretty self-evident to me.

Doesn’t QT belong on your list? Or am I missing something?

I'm not writing mobile applications in C++.

Genuinely curious: In what way is Kotlin Native severely compromised?

The reason, to me, to use Kotlin is to leverage the JVM without having to write Java or Scala. When you don't have the decades of quality JVM libraries there, I don't see much value in Kotlin, outside of high-performance applications that I don't need to write on mobile. Instead I can write TypeScript--and while I don't hate Kotlin, I quite like TypeScript--without having to constantly screw with marshaling between the native/UI layer and Kotlin Native.