Comment by diego_moita

For Windows only (because that's the most common platform in the world): C# and .Net. The UI is mostly either WPF or WindowsForms.

For Mac only: new apps use Swift, legacy ones use Objective-C, both with AppKit.

Linux only: these are quite rare and there is a broad mix of them: C++, Java, etc.

Cross-platform: C++ with Qt seems to be the standard. Go and Rust seem to be rising. Rising UI libraries are Tauri, Slint and Iced. There are other alternatives like cross-platform .Net, Java or Flutter. But they don't look very solid and widely adopted.

Then there are other UI frameworks for some specific use cases (games, scientific applications, etc).