[teh_klev]

The thing you're thinking about is http.sys and it was introduced in Windows 2008/IIS7 to support the new "Integrated Mode" request pipeline.

WinHTTP doesn't have anything to do with http.sys it just listens for HTTP requests and then hands them off to the right bits inside IIS.

WinHTTP is essentially a HTTP stack for client services running on Windows Server to allow them to make HTTP requests. It has a sibling API named WinINet which is aimed at use in desktop environments. I think the threading models are main differentiator (I been a while since I looked at this).

WinHTTP is fairly well documented:

https://learn.microsoft.com/en-us/windows/win32/winhttp/abou...