When I contracted with them we ran our computation loads on a Linux server and deployed our service to an internal Linux server. Only osx I touched was my laptop, and that was a work requirement they insisted on.
14 years ago the answer was Oracle Linux, based on what I was told by my boss who left us to take a job at Apple
not sure what flavor / distro they use nowadays but almost certainly Linux and almost certainly one with an Enterprise Support agreement.
Linux, just like Microsoft’s.
Just like every other cloud.

A lot of them genuinely ran Mac OS Server back when that was a thing. Probably some that still run macOS, but now most probably just run Linux.
I ran a cluster of XServes back in the day. Probably some of the most well engineered hardware I had seen since Solaris Sparc’s in the 90’s. That said, the software stack didn’t perform well. Ever. Way too much overhead.
Now that is interesting. I know Windows Server exists for small enterprises, but I didn’t think MacOS Server would be something used for larger enterprises. Figured Apple would’ve used AIX in the days of yore.
Which servers? (But the answer is almost certainly Linux.)
BSD or Linux like everyone else’s.
Likely a modified version of MacOS which is a modified version of BSD.

