Everything I've been able to find has been about iCloud, but what I want to know is: "How do Apple manage their internal IT?"

I find it hard to believe that everyone at Apple are just managing their own devices and use iCloud for filesharing. They might, but it seems weird if they don't have a need to at least some internal servers, where they'd be forced to run either Windows, Linux or some BSD. For device managing I guess they could be using their MDM service.