It appears they use at least some amount of Super Micro hardware, possibly in very generic or custom cases (ie, they're not buying Dell.) That being said, I have to wonder how much datacenter space Apple really has. IIRC most of their iCloud services are hosted on various 3rd party cloud providers.
https://www.macrumors.com/2017/02/23/apple-ends-relationship...
https://9to5mac.com/2021/02/12/super-micro-spy-chip-story/