First thought: something like compose-navigation wouldn't be available on desktop, right? That probably means we would have to build abstraction layers if want to build some cross-platform functionality. Is there a recommended way, through Ambients or something else?

I'm guessing there will be some kind of layered approach. Components that apply to all platforms. Mobile components. Desktop components. Web components. Then in your common sourceSet you only have access to truly common components and on your platform sourceSet you get a superset of those

Hope they just port the android navigation to avoid having multiple libraries for the same purpose.

It can be the same library with expect actual that's typealiasing android compose stuff

It is already decoupled but there is code in Java

Decompose works just fine) https://github.com/arkivanov/Decompose

not like "fragment" fragments, right?

Not sure what you mean) the idea is same but with proper DI and pure Kotlin

oh, it is like the Fragment class, kind of, I guess. Hmm interesting, but it manages to avoid the pitfalls that Fragments fell into?

Yes it is designed with Fragment's problems in mind. So basically lifecycle-aware components with routing.