In the documentation there are many articles about using C types & functions from Kotlin, but I didn't see an article to use Kotlin types from C (eg. Making a library in Kotlin/Multiplatform and using it in C code). Is it possible?

Yes. https://github.com/JetBrains/kotlin-native/blob/master/FAQ.md#q-how-do-i-create-a-static-library-or-an-object-file

Thank you