Kotlin provides two killer features: multiplatform projects (including common projects) and coroutines. But multiplatform doesn’t provide any special functionality by itself, only lets you to use the exposed APIs from each platform. And coroutines are low-level too.

These libraries expose a common standard API for multiplatform projects: exposing atomic operations working in all platforms, providing coroutine-aware asynchronous tools, or being able to serialize and deserialize from one platform to another.

Kotlinx.* (Kotlin eXtensions) libraries are OpenSource and you can find them at https://github.com/Kotlin/