> We understand that Basecamp has developed a number of apps and many subsequent versions for the App Store for many years, and that the App Store has distributed millions of these apps to iOS users. These apps do not offer in-app purchase — and, consequently, have not contributed any revenue to the App Store over the last eight years.

They literally have paid 8 years of Developer Program fees for supporting the AppStore.

It's a different matter whether 30% is a valid cut for Apple to make on Hey In App subscriptions. But $2,400 (8*$300/year?) for the delivery and update, notifications and Siri integration etc to millions of apps/devices sounds a pretty good deal.

+ the hardware mandatory to build and submit Apps to the AppStore, right?

Yes. Apple forces you to buy their overpriced hardware if you want to develop for their platform.

Xcode comes with an iOS simulator so you don't need hardware, but it would be a bad idea not to test on an actual device.

How do I run xcode on Linux or Windows?

In qemu.

Is this legally possible without software piracy? Can you link me to instructions? Does apple have documentation anywhere?

https://github.com/kholia/OSX-KVM

I don't know if Apple has documentation somewhere.