Free BSD
https://www.freebsd.org/about.html
People still use that ?
What is FreeBSD?
FreeBSD is an operating system for a variety of platforms which focuses on features, speed, and stability. It is derived from BSD, the version of UNIX® developed at the University of California, Berkeley. It is developed and maintained by a large community.
UNIX was a Bell Labs product donated to the UC Berkley as open source. Bill Gates used the IP stack they developed for windows.
I never used iOs because closed source UNIX derived software is the devil's work.
