All three frameworks will be made available on GitHub. Developers will be able to contribute to in Microsoft's client UX technology, as well help debugging and fix issues. They also can build a private copy of the UI stack.
The open-sourcing of the three frameworks serves as a followup to having the open source .Net Core 3 framework support WPF, Windows Forms, and Universal Windows Platform (UWP) XAML, so existing and new Windows applications can run on .Net Core.
WPF is a UI framework for building desktop client applications, with an application model, controls, graphics, layout, data binding, and security. At the core of WPF is a rendering engine to take advantage of modern graphics hardware.
Windows Forms enables development of “smart clients,” described by Microsoft as graphically rich applications that are easily deployed and updated. These applications can work whether or not they are connected to the internet. A form in Windows Forms is a visual surface for displaying information to the user. WinUI has backward-compatible implementations of the default UWP XAML UI platform for Windows 10.