.NET (not .NET Framework) is cross platform and can be compiled into native binaries on a variety of platforms. There is however the wrinkle of not all the libraries within .NET being supported on all platforms. Most notably, everything involving a graphical UI is Windows only.

The most well known cross platform .NET project you probably have heard about is Jellyfin.

It's a lot better with some notable exceptions. First, .NET Core is multiplatform by design, so it is by default quite portable. The .NET Core CLI is extremely powerful and means a CLI workflow is totally feasible (and also simplifies CI pipelines). The new "multiplatform" application framework, MAUI, runs on Windows, Mac, iOS, and Android, but not Linux/GTK/QT etc. You can maybe attribute this to the design philosophy of abstracting native controls, of which "Linux" itself has none, but either way it's useless on Linux. Third party frameworks like Avalonia do work very well on Linux.

Modern .NET (i.e. .NET Core and later) is cross platform. In fact, .NET APIs now are routinely run in containers not based on Windows.