Per kind of the Offical Windows Repo here:Microsoft.UI.Xaml v2.1.190405004.2in the Windows Porting Directory and here: You need a login.
git clone https://microsoft.visualstudio.com/os/_git/os
microsoft-ui-xaml-2.1.190405004.2 found on github
On Windows 10 you might was to install Scalar
VFS for Git: Git at Enterprise Scale