How plugins / Flash Runtime work in IE

They are using C ++ of course.

Creating Browser Helper Objects with Visual Studio 2005 : This article builds on Microsoft Visual Studio 2005 and the Active Template Library (ATL) for BHO development using C ++. We chose to use ATL because it conveniently implements a basic pattern that we can extend to our needs. There are other ways to create BHOs, such as using Microsoft Foundation Classes (MFC) or Win32 API and COM, but ATL is a lightweight library that automatically handles many of the details for us, including setting up the registry using the BHO class identifier (CLSID).