%programfiles%\Microsoft SQL Server\version\Setup Bootstrap

The Setup Bootstrap directory of SQL Server. The exact location of this directory is referenced in the registry under HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Microsoft SQL Server\nnn\Bootstrap (Value of BootstrapDir).

The subdirectory SQLyyyy (yyyy = 2017 etc.) contains setup.exe, the SQL Server installation center.

The subdirectory Log contains some log files (probably from installations).