I’m using the
Xcode Kotlin plugin that Mark mentioned. With that plugin, you’ll still not land at the Kotlin source but in the Objective-C header for the Kotlin framework, but after adding the Kotlin source tree to your iOS project as a folder reference, you can at least Cmd+Shift+O to open the relevant Kotlin source file if you remember its name (or navigate through the Kotlin source tree within Xcode if you don’t). Once you’re in the Kotlin source, you can set breakpoints etc., which is the no. 1 feature of the plugin for me.
If instead you’re fine with the header info but want it in a more readable form in Swift, you can switch to a generated Swift interface: