My understanding is that internally when they have tasks that their own hardware isn't appropriate for they use a combination of Linux and macOS running on commodity PC hardware. They have an internal version of macOS for the commodity PCs. The person who told me hinted they got machines from a big name vendor like Dell or HP. I was informed of this in a discussion about AR and VR. Basically we all asked how Apple even did work on this internally when their own machines clearly had some issues with that sort of workload. This was like six months ago and related to speculation about showing the new Mac Pro at WWDC 2018, which didn't end up happening.