Visual Studio is not written in C#/WPF, it’s written in a mess of legacy COM and WIN32 code with some C# and SWF.
(Visual Studio for Mac is, of course, an entirely different product and codebase).
(Visual Studio for Mac is, of course, an entirely different product and codebase).
The brand name has been re-used for other software. Visual Studio for Mac is Xamarin. Visual Studio Code is on the Electron platform.
”For a long time now developers have been asking why Visual Studio hasn’t made to switch to 64-bit. Rather than effort or opportunity cost, the primary reason is performance.” https://www.infoq.com/news/2016/01/VS-64-bit
The new plugin architecture, replacing the COM one, is implemented in .NET.
Visual Studio for Mac has many new features that weren't present in Xamarin Studio, like .NET Core support, ASP.NET types of workflows. And all new Xamarin.Forms improvements are shared between both IDEs.