Is there a Kotlin Multiplatform JSON library? I've read about Ktor and from what I understand it uses a different engine per platform, but it doesn't provide any common API so as soon as JSON handling happens it needs to be platform-specific?

Yes there’s https://github.com/Kotlin/kotlinx.serialization. See also #serialization

Thank you

Is serialization runtime with reflection?

It’s not reflection on native for sure. I think it’s a compiler plugin