NeXTSTEP technologies continue to influence Apple devices like Macs, iPhones, and iPads 16 years after Apple acquired NeXT in 1996. Underlying OS X and iOS is a powerful, certified UNIX operating system, providing features like protected memory, pre-emptive multitasking, and daemon-based services. Objective-C, developed for NeXTSTEP and used in OS X and iOS, revolutionized software development. The AppKit framework from NeXTSTEP evolved into Cocoa for OS X and Cocoa Touch for iOS, simplifying app development. Display PostScript, Interface Builder, Bundles, Column file browsing, and Services are additional NeXTSTEP-based features integrated into OS X and iOS, enhancing user experience and functionality.
https://arstechnica.com/gadgets/2012/12/the-legacy-of-next-lives-on-in-os-x/