how would a mobile/desktop multiplatform app work with Kotlin + Jetpack Compose? Say, targeting iOS/Android/Windows/Mac? Business logic in Kotlin, Compose for everything except iOS, and then write the iOS view layer with Swift UI or whatever?

I think yes. You can check the TodoApp sample, it is designed exactly this way. https://github.com/JetBrains/compose-jb/tree/master/examples/todoapp

wild. thanks

Not exactly, there is also JS which is not yet supported by Compose, and all other Native targets. But this might be a benefit, because you use native UI. Everything else can be shared, from data layer to and including navigation.

True, but I excluded JS from my list intentionally since it is itself an odd duck in a way. People use web apps from either mobile or desktop, so not having a web app in that case isn't particularly exclusionary

There were open roles to work on Web Compose AFAIK, plus there were some related commits.

Yeah i believe a thread earlier in the year referenced them working on Compose for JS.

Current web-compose uses DOM widgets, not Android widgets, so it is a little bit different (I would be happpy with DOM).