Kotlin language is (one of) official language for Android developent
and it can also be used for building "native" apps, including integration with iOS apps, similar to way C# with Xamarin i used for Andorid and iOS apps.
Since Kotlin is newer language, with simple syntax and powerful features,
it an interesting tool for multi-platform development.
Tutorials - Kotlin Programming Language
Multiplatform Project: iOS and Android - Kotlin Programming Language
- Create an Android app with Android Studio
- Create a shared Kotlin library
- Create an iOS app with Xcode