How are these crates built?

The windows and windows-sys crates are generated from metadata describing the Windows API. Originally only WinRT APIs included metadata, but metadata is now provided for older C and COM APIs as well. The win32metadata project provides the tools to produce the metadata and the windows-metadata and windows-bindgen crates are used to read the metadata and generate the windows and windows-sys crates. The bindings are generated differently based on the differing goals of the respective crates. You can find the exact metadata files used to generate a particular version of the windows and windows-sys crates here.