I worked on some of the internal server hardware. Yes they do have their own boards. Apple used to be all-in on Linux, but the newer chips are far and away more power-efficient, and power is one of the (if not the) major cost of outfitting a datacenter, at least over time.
These machines are very much internal - you can cram a lot of M-series (to use the public nomenclature) chips onto a rack-sized PCB. I was never under the impression they were destined for anything other than Apple datacenters though...
As I mentioned above, it seems to me there's a couple of feature that appeared on the customer-facing designs that were inspired by what the datacenter people wanted on their own PCB boards.
Are these internal servers full of M-series chips running a server max osx build then as well?
Apple's OS builds are a lot more flexible than most people give them credit for. That's why essentially the same OS scales from a watch to a Mac Pro. You can mix and match the ingredients of the OS for a given device pretty much at will, as long as the dependencies are satisfied. And since you own the OS, dependencies are often configurable.