macOS is a proprietary operating system developed and released by Apple to be used on its Mac/Macintosh line of Apple products. macOS, previously Mac OS X and subsequently OS X, is fundamentally based on Apple’s open-source Darwin operating system, which in itself is based on NeXT’s Unix-like NeXTSTEP operating system.
macOS has a storied history beginning at NeXT, Steve Jobs’s company following his leave from Apple, where the NeXTSTEP operating system was created in 1989. Afterward, when Apple bought out NeXT, they used NeXTSTEP to serve as a foundation and basis for its next generation of operating systems. After the creation of Mac OS X, Apple also created an open-source project which included contents from many foundational elements of macOS, called Darwin. In 2012, coinciding with the release of version 10.8 named Mountain Lion, Apple shortened the name of Mac OS X to simply OS X. The next major change in the operating system came in 2016 with the 13th major release (10.12) named Sierra, resulting in another name change from OS X to macOS, reflecting the camelcase like capitalization branding of its other operating systems. (iOS, iPadOS, watchOS, tvOS, bridgeOS, audioOS) In 2020, 20 years after the original release of the Mac OS X beta in 2000, Apple announced macOS 11, marking the end of the version 10.x macOS generation, along with adding support for ARM processors and new iOS-like icons and a new design.
macOS is designed and regulated to only run on devices and hardware developed and manufactured by Apple. During the majority of Mac OS X’s early lifetime, it supported PowerPC up until Mac OS X Leopard. (10.5) Following the drop of PowerPC support, the majority of version 10.x of Mac OS X/OS X/macOS, only supported Intel x86 based processors. With the release of version 11.x macOS operating system, Big Sur, support for “Apple Silicon”, the trade name created by Apple for it’s ARM-based SoCs (System on Chips) and SiPs. (System in Packages)
While there are workarounds, devices that run macOS on non-Apple devices are few and far between. Computers running macOS on “foreign” or hardware not developed by Apple are referred to as Hackintoshes by the community.