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kotlinlang

Is "multiplatform" just a synonym for "iOS" at Jetbrains? I see very little evidence of them treating MS Windows as a first class platform.

KMM is mainly about iOS and Android, yes. But KMP is much wider and applies to Mac OS, Linux, Windows, Web and so on.

on platforms that can run a JVM, you don't need KMP to make it possible to run Kotlin code, and you get a more developed ecosystem too. so outside of iOS and web, I don't see a huge demand for multiplatform

Kotlin JVM is cross-platform for desktop (it runs on Linux, Windows, MacOS, ...), with no additional configuration. Kotlin Multiplatform is a technology to share code between platforms that traditionally can't be shared with (Web via JS, Native for iOS and embedded systems).

KMM is a subset of Kotlin Multiplatform that focuses on Android + iOS (because that's what people want the most, so that's what they're focusing on, but full on Multiplatform is already in alpha and will become beta etc at some point)