It can target JVM (windows, Linux, Mac) and then work on iOS and Android.
Android and JVM are stable. IOS is alpha and works well. Should be beta this year.
WASM support is coming as well but is experimental.
You can do as much multiplatform as you want and do as much platform specific as you want.
Compose itself is a declarative UI framework. Your UI is code.
Edit: You do require a Windows, Linux, and Mac machine to build the executables for each desktop JVM app, as well as a Mac for an iOS app. Android you can build on any of them.