Complex sentence: These extensions are a large part of what makes windbg such a powerful debugger-it is used by the product team to build windows and everything needed to debug windows is baked into these extension dlls.
Split into simpler sentences: These extensions are a large part of what makes WinDbg such a powerful debugger. WinDbg is used by the Microsoft Windows product team to build Windows, and everything needed to debug Windows is included in these extension DLLs.