MacOS (Macintosh Operating System) is the operating system that Apple computers use. It is developed in-house and is proprietary software. MacOS since the introduction of Mac OS X is an unix-like operating system based on darwin. While darwin is open-source, MacOS provides many additional layers that are usually very complex and are not open-source, such as the Quartz Compositor. MacOS is a certified UNIX operating system.
History
The history of MacOS is quite a weird one because the original operating system is not the one in use today anymore but rather a completely different one.
Classic Mac OS
Initially, MacOS wasn’t even called MacOS but rather “System X”, where X represents the system’s version number. This was done to blur the lines between the operating system and the hardware. Apple has done this to make it more beginner-friendly to use the system. The whole system should be considered as one by the user.
Unlike other operating systems at the time, MacOS provided an abstraction over the hardware layer, which allowed applications to be more portable. This means that if an application was developed for an older system, it probably would work on a newer one because it only interacted with the operating system and not with the hardware directly.
Another thing that seperated MacOS from other operating systems is the lack of a CLI. The computer was ment to fully be used only by GUI. This was unheard of at the time and many concepts like drag and drop originate from this operating system.