All desktop builds that were released as Windows Insider builds are included, but some simply don't have any icon changes (e.g. Build XXXXX). Builds without changes are hidden by default on the builds page, but can be shown using the toggle at the top of the page.
Leaked builds that were never officially released (some partner builds, for example) aren't included at all.
The Windows 10 icons database tracks all .dll and .exe files in %windir%\system32\, files that don't have any icon resources inside are naturally not listed on the site, neither are files that do have icons inside, but whose icons were never updated since Windows 8.1. These files are however still tracked behind the scenes, they become available online as soon as an icon gets added or changed in these files.
All icons are extracted from 32-bit English-US Professional builds.
Builds aren't installed for the extraction process, they are downloaded as ESD file and then extracted. Dll and exe files are pulled directly from the extracted folders to avoid contamination by other software/hardware.
The builds are downloaded as ESD file and extracted using a modified version of ESDDecrypt by abbodi1406 & other MDL members and wimlib by Eric Biggers. The rest of the process is automated using a custom Python script, it uses IconsExtract to extract icons from dll and exe files and NConvert to convert icons to png. The script further relies on Pyton's filecmp module for comparing the icons and ftplib and pyodbc for uploading them and adding them to the database.
Probably not. All the icons on this site are shipped as part of Microsoft's Windows 10 operating system, all the copyright therefore belongs to them.
I have no idea. I haven't heard of any Microsoft employees talking about The Windows 10 Icon Database, but I haven't gotten a letter from their lawyers either...