macOS (previously Mac OS X and OS X) has gone through multiple iterations since it was first introduced in 2001. Prior to that, a series of operating systems developed between 1984 to 2001 for the Macintosh family of PC was in use; now, they are known under the colloquial term classic Mac OS. macOS belongs to the family of Macintosh operating systems derived from NeXTSTEP. This operating system was originally based on Unix (particularly, BSD with the Mach microkernel). Using a Unix-derived architecture was a completely new direction compared to the previous Mac OS solutions.
Apart from traditional C/C++ languages, the main programming languages that Apple supports in their products are objective-C and Swift (since 2014). Interactions between applications and the OS are possible through the native API, called Cocoa, derived from OPENSTEP; prior to that, Carbon API was used.
There are multiple mechanisms implemented in the operating system that aim to boost security while always keeping...