String.kt for example will use String.java on the JVM?

That depends on the type and the platform. For Strings you will get

String.java

on the JVM and a plain

string

in JS for example. But if the target platform does not have a native version of the type, it will be emulated (

Long

for example is emulated by a class containing two 32 bit numbers on JS, because JS only has a 64-bit floating point).

thanks

There is a section in the documentation about this: For JVM: https://kotlinlang.org/docs/reference/java-interop.html#mapped-types For JS: https://kotlinlang.org/docs/reference/js-to-kotlin-interop.html#representing-kotlin-types-in-javascript

ah thanks I remember that

(I am sure there is something for native, too, but I am not experienced with that)

thanks for the link