It's indeed not a fully native application except on Android. It uses JetBrain's 'Compose Multiplatform' framework for the Kotlin programming language. Without having written code in it myself, it's difficult for me to work out exactly what its architecture is, but it seems closer to Flutter's approach than a web wrapper. My educated guess is that it renders the application inside an OpenGL/Metal graphics context. On Android, however, it looks like it uses Google's Jetpack Compose components directly.