Unix and C
Bell Labs after Multics, Thompson and Ritchie's Unix (1969), C and the rewrite (1973), pipes and the Unix philosophy, licensing and the spread to universities, BSD and TCP/IP, the Unix wars, GNU and free software, Linux, open source as a development model, and macOS and Android as Unix descendants.
Small tools, portable code
Unix's design (everything is a file, small tools connected by pipes, text as the universal interface) and C's closeness to the machine shaped almost all later systems. macOS descends from BSD through NeXTSTEP; Android runs on the Linux kernel; the servers behind most websites run Linux. Thompson and Ritchie received the Turing Award in 1983 for the development of Unix and operating systems theory, and the US National Medal of Technology in 1999.
Dennis Ritchie, "The Evolution of the Unix Time-sharing System" (1984); Brian Kernighan, UNIX: A History and a Memoir (2019); Peter Salus, A Quarter Century of UNIX (1994); Linus Torvalds and David Diamond, Just for Fun (2001).