The development of OS X (MacOS 10) was a vary complex and long process that goes all the way back to 1985. In 1984 after the first Macintosh was released Steve Jobs was kicked out of apple, when he was away he created NeXT I made a post about OpenStep but a recap is: NeXT made computers that ran Nextstep, Nextstep is a lot like OS X but in 1988 the only problem is that these computer sold for about as much a Mac Pro with all it’s accessories so NeXT created OpenStep that could run on most computers you could also build on to the OpenStep technology to create your own OS like GNUstep. When apple bought NeXT Steve Jobs became apple’s C.E.O again, Steve Jobs made MacOS 8&9 to keep macOS alive and created Mac OS X Rhapsody DDR 1&2 Rhapsody was built on MacOS 9 and OpenStep and after Rhapsody was created apple made the OS X development preview with DD1-4 DDR 3 was the first development preview to introduce the aqua interface. In Mac world 2000 Steve Jobs announced the release of the OS X public beta for testing, soon after OS X 10.0 Cheetah was released after that apple focused on small improvements to macOS. Please come back to eddy4ai.com, Thanks for reading, Edward.