Those comments auto-build an XML output; it's a built-in feature of the C# compiler. [1]
There are a number of XSLT stylesheets and more complicated tools around, many from relatively ancient times in the .NET world, to generate prettier documentation from those comments. NDoc [2] is ancient but still presumably works. I last used SandCastle, whose website proclaims it shutdown and forwards to fork SHFB [3], and some of whose guts inspired DocFX [4].
As another comment points out it appears that DocFX is the latest toolset Microsoft themselves are using (it's one of a couple of build tools for the new mostly all open source docs.microsoft.com site).
[1] https://docs.microsoft.com/en-us/dotnet/csharp/programming-g...
[2] http://ndoc.sourceforge.net/
[3] http://github.com/EWSoftware/SHFB
[4] https://dotnet.github.io/docfx/