Kotlin Multiplatform allows you to work fully native on Android as Kotlin is the main language there. And it also allows to compile to native iOS code and integrate with native iOS Objective-C code or bridge code. (Swift integration is on the roadmap) It also allows you to directly call C code or C libraries. Or it allows to compile to JS and call JS code. So it is more versatile than Flutter and easier to integrate with native code/components. Flutter is more creating its own world with the Dart sandbox and then has to pass messages to communicate with the native world. Kotlin is closer to native output.

Jetpack Compose is the UI library and provides a similar mechanic as React/SwiftUI/Flutter in building declarative UIs. It is already the Android default as Google started to develop this UI library and Jetpacks brought in the multiplatform implementation.