I thought it was the other way around? C# on the outside, legacy code connected on the inside? Anyways, I found that confusing. For the rest, visual studio most definitely uses WPF for much of the UI but also the chrome (e.g. see https://softwareengineering.stackexchange.com/questions/7532... and https://www.quora.com/As-of-2015-how-much-of-Microsoft-Visua... and https://blogs.msdn.microsoft.com/visualstudio/2010/02/16/wpf...).

I'm quite sure the IDE is WPF. You can attach a debugger to it and inspect the WPF component tree.

You're both right. Yes, it's WPF, and there are lots of parts written in C#. But the parent is right -- Visual Studio is C++ using COM and Win32 at its core, going back all the way to the Visual Basic IDE that shipped as part of Visual Studio 6. It would be a very difficult port to Mac, Linux, etc. As of this writing there is even no 64-bit version for Windows (although that's just a matter of spending the time and money to do it.)

The brand name has been re-used for other software. Visual Studio for Mac is Xamarin. Visual Studio Code is on the Electron platform.

Surprisingly, the lack of a 64 bit version is a performance choice.

”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

All the new code in Visual Studio for Mac for .NET tooling is being shared with Visual Studio, written in C#.

The new plugin architecture, replacing the COM one, is implemented in .NET.

I thought VS for the Mac was just a rebrand of Xamarin Studio?

Initially yes, but they have been making the Roslyn tooling the same across both IDEs.

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.