Apple itself when I worked there was strict on having no gpl3. But everything on the inside is Linux. They even have their own version of arch (and Java and spring boot which is what I did)
You think iCloud is off Mac minis?
Server workloads are almost universally run on RHEL of some variety, presumably because (as sibling post mentions) of long term support contracts.
Ironically, I was involved in the effort to port swift to linux so you'd think I'd know. The team I was on eventually moved most of our work to Rust (it was, also unusual for apple, written in C before).
[1] And anyone who would ever come into HN and tell you anything about what goes on inside apple could never tell you anything with that level of certainty because even in Services where things are generally more open, stuff is still very siloed and fragmented and no one has a complete picture of what's out there.
Not even Eddie?
But i suppose if he made a top down order to enumerate things he might get a good idea. But Apple is just.. kinda weird. Things can be a secret even to the reporting chain above the people working on them. Certainly the specific tech being used can be closely guarded.
And then there are just several different infrastructure systems things are on, with varying degrees of openness, and most of them designed to resist tampering or unplanned inspection.
Maybe BSD would fit better with their lineage, but Linux has far better enterprise vendor support.
It's a beautiful fantasy, though.