At my current place of work, we have a rack of Mac mini’s in a closet as our build server for the iOS app. I was kind horrified and amazed to learn of this at first, but it really is normal. It’s impressive what gymnastics Apple compels us to go through :)

> At my current place of work, we have a rack of Mac mini’s in a closet as our build server for the iOS app.

One of the main selling points that Apple uses to sell Mac minis is their use in build farms and render farms.

If you click on Apple's page on Mac minis and scroll down until the 6th or 7th selling point, you'll end up seeing the part where according to Apple it's combination of stackable form factor and performant hardware makes it ideal to compile large xcode projects or perform video encoding.

I wonder if Apple is building their OS and Apps on Apple hardware or if they have real servers in their data center.

It must be quite common, or companies wouldn't sell dedicated rack mounts for these:

https://www.sonnettech.com/product/rackmacmini.html

Minis are also common in homelabs. Small, cheap when used (even cheaper if it's your own old machine), and natively runs a Unix-like environment for each management and can easily boot into Linux instead. I suspect the switch to Apple silicon is going to change that, however.

I could see that switching to Intel Nuc and Intel Clear Linux instead.

That shift already seems to be happening. If you follow /r/homelab, plenty of people are building large homelabs using nothing but NUCs they paid $250 each. It's not uncommon to see racks with 4-8 of them networked together (you usually only see 1-2 Minis). There are also some AMD-based machines coming out that are that size, so there will be even more options available.