I came here to ask the same thing. So if Apple don't make a rackable M1 server, what do they themselves use? It seems a bit unlikely Apple would have racks of minis as well.
According to a sysadmin I interviewed several years ago, they use standard x86 server hardware from other vendors like Dell. If I recall correctly (there's a good chance I'm misremembering), they ran something like VMware, and ran Linux and OS X in VMs.
Oh right: Apple isn't constrained by their own license terms, so there's no reason they couldn't run Mac OS virtual machines at any scale on any hardware they wanted. Up to and including things that you would otherwise probably call "perfect Hackintosh" builds, since they can build actual Mac OS images with the hardware checks disabled, paravirtualization drivers, whatever.
When I worked as a contractor at Apple Retail Software Engineering, they bought for themselves the full final couple of shipments of thousands and thousands of the rack-mount Macs they had designed and built, and then ultimately killed. They filled a couple of data centers with them. And then they ran VMware on them so that they could have a lot more virtual Macs as servers.
They were also buying rack mount hardware from other vendors and running VMware on them, too. But I don’t remember what the vendors were that they were buying from. I doubt it would be Dell, given what Michael Dell said about Apple years ago, and the rivalry that has existed ever since. But I don’t know for sure.
You can literally buy a rack count tray for Mac Minis on Amazon, so I’d assume AWS mounts them in racks. It’s not the cleanest model, but Mac mini’s are smaller than 1U (1.4” vs 1.75”). They are also much cleaner to rack than the old trash can Mac Pros.
(Non affiliate link to a randomly chosen example)
Mac Mini Rack Mount, UCTRONICS 19" 1U Rackmount Supports 2 Units of All Mac Mini M1 and The Previous Models