Facebook engineers (most notably Sam Gross) contributed a lot of the no-GIL work: https://lwn.net/Articles/939981/

Much of the initial work on the JIT came from Microsoft's Faster CPython team: https://lwn.net/Articles/1029307/

Lots of people. Several people from Arm and Microsoft, various PhD students... I don't know if anyone working at Facebook worked on the JIT, maybe they did.