I thought C++ and C# introp was available with KN

Only C at the moment, cpp has some fetures which are too complex to map in Kotlin. C#? No plans to that according to my knowledge

C# is not native, it's closer to Java and has own virtual machine CLR

Like Java, it's possible to compile it to machine code with caveats, do not recommend. But anyway... it's possible to do C and ObjC with minimal effort because they are "C compatible" - the term typically used escapes my mind. Compiled Swift might work with cinterop, but for C++ you'll probably need to make a wrapper, which is fairly simple. You just make functions that take a pointer as the first parameter which would be a pointer to the object, and you could create wrappers in Kotlin that hold a 'val ptr: Long'... tldr, cpp first class interop might not be worth the hassle, but I have a production app that uses C++ with Kotlin and Swift, so anything is possible with a little effort.

Well, for small C++ lib effort is little, for huge libs like Qt - it's not possible.