Flutter and Dart are used on a plethora of platforms, including Android which has a far larger market share than iOS alone, and Windows, which takes the cake on the desktop.
Swift is used only on Apple platforms, and mostly for iOS. The use of Swift on macOS, watchOS or server-side development is negligible.
The first is Google trends results for Flutter vs Swift globally in the last year.
https://trends.google.com/trends/explore?q=%2Fm%2F010sd4y3,%...
Along with StackOverflow’s 2021 report that shows Dart is more popular than Swift.
https://insights.stackoverflow.com/survey/2021