The Skip transpiler runs locally on every build. It is implemented as an Xcode build plugin, so it is transparent and instantaneous. Every change you make to the Swift code is immediately converted into Kotlin, so each time you launch the app in the iPhone Simulator, the Android app will launch next to it in the Android Emulator. For a good overview of this process, take a look at: https://skip.tools/tour/skip-showreel/