C# has had cross platform support for 17 years. Without getting into the horror show of absolutely counter-intuitive acronyms that is modern .NET development (of which C# is a part), .NET now directly and natively supports cross platform targeting. I can build a Linux/Mac/etc binary from my Windows machine with the same compiler by changing a single flag. It's pretty cool stuff.

The Uno platform is a third party project that ties this, and more, together to enable building single code-base cross platform apps with native UIs simultaneously targeting Windows/iOS/Android/Web (WebAssembly)/Linux/macOS/Surface Duo.