When they relaunched the desktop/laptop apps in 2019, they were built on Electron as per Nate above. On my Mac the desktop app opens certain links with “Electron App” listed as UTM source:
?utm_source=Electron%20App&utm_medium=Mac&utm_content=Terms%20and%20Conditions
The iOS beta app has a link to licenses:
which looks like Xamarin and ReactiveUI are key components of the iOS and Android apps. But my background is computational algorithms, networking, and operating systems.