Delphi still exists and you can still do it in Delphi. Responsive. Cross-platform on Android, iOS, macOS, Windows, and Linux. Single codebase single UI.
Can you point to any Delphi app on the android playstore that doesn't look and feel like shit? Or a Linux app? I used to be a Delphi fan, but the results I've come across are not exactly convincing.
Compare this to Flutter, which looks and feels gorgeous, at least on Android.