> I thought OS X was tied in some way to Apple hardware (i.e. you can emulate the CPU but they tie it to other hardware).

It is, but very weakly - basically a secret string stored in the SMI.

> Is this how Github actions and other CI services work? For some reason I thought they had to buy a bunch of Apple hardware ...

You can virtualize MacOS, but only on official Apple hardware. There is likely a fleet of Mac Pros or Mac Minis somewhere running real hardware + VMs.

EDIT: Read "You can ..." above as "You are authorized by Apple and can legally virtualize MacOS". You can run OSX on virtually any hardware in a VM, but doing it at scale or in a commercial setting will probably be a bad idea from a legal perspective.