We've had Kotlin Multiplatform for a while now, so a developer writing an Android app in Kotlin could bring it over to iOS and desktop platforms today with reduced effort. Compose Multiplatform aims to take this a step further by allowing UI code sharing instead of just business logic.

The future is not that far off where you can write a 100% native Android app and, minus any Android OS-specific functionality, port it over as-is to iOS and desktop.