Windows was originally just a Graphical user extension to Microsoft existing product MS-DOS. Most operating system under the windows brand had been based on the MS-DOS architecture (apart from NT). Windows XP had been the first home consumer operating system to be shipped to customers that had been based on the NT version of windows. To this day, windows is still based upon the NT architecture.
Apple’s operating systems are built upon NeXTSTEP that was created by Steve Jobs during his absence from Apple. Upon Steve Job’s return to Apple, NeXTSTEP had been bought by apple and the NeXTSTEP operating system laid the foundations for all of Apple’s operating systems (macOS, iOS, watchOS and tvOS).
