Those are all built on the open-source-based Darwin kernel.  
Aqua is what everyone sees.  Aqua is built on Classic, to run programs native to 8.1 or later, Carbon, to run the new programs, and Cocoa, for execute Java-based programs.  Those are all built on the open-source-based Darwin kernel.  FTP identify this strange flavor of Unix that saves documents in Adobe PDF, runs graphics through OpenGL, and is fully QuickTime compatible because it is created by the makers of QuickTime.
Aqua is what everyone sees.  Aqua is built on Classic, to run programs native to 8.1 or later, Carbon, to run the new programs, and Cocoa, for execute Java-based programs.  
Mac OS X (accept: Mac OS 10)