In my common modules I have a Retrofit interface which I manually implement for JS and then use Retrofit on JVM

On Native (via Kotlin Native) Lib Curl ( https://curl.haxx.se/libcurl/ ; https://github.com/JetBrains/kotlin-native/tree/master/samples/libcurl ) is the defacto HTTP client library (also support other protocols like SSH, SFTP, SMB etc).

Yeah the problem is never which HTTP client to choose, it's the lack of support for the reflective mechanisms that Retrofit relies on on each platform (at least through Kotlin)