I figured they would have an internal build of macOS for servers, seems like the logical thing to do.

It seems cheaper to just sign a large service contract with a company for which building a server OS is the core competency. Apple is very much not an engineering-first company. They make very logical decisions from a business-first point of view.

They don’t want to port all that software to a secret OS that isn’t supported upstream. Also much of apple, like I was, were contractors. So your there for a bit to do a thing. People know Linux so that’s what they use. Honestly much of what I did was on Amazon Linux 2 because it was just their own custom AMI on EC2

I feel almost disappointed they don't at least run FreeBSD, but I guess it makes sense. They make a desktop OS, and most of the talent on the market has used Linux.

Well, in the old days Apple did use some Xserves.