Leaked files uploaded on BetaArchive website contains more than 32 terabytes of data. Microsoft confirms that these files are actually a portion of the source code from the Shared Source Initiative and is used by OEMs and partners. Windows 10 internal builds include private debugging symbols defined by the engineers to help other in-house developers understand how some specific codes in the operating system works and what functions it calls. The leaked files also contain, a private software toolkit created by Microsoft designed to run Windows 10 on mobile devices.
Source: https://thehackernews.com/2017/06/windows10-builds-source-code.html