Lots of other replies about how Apple software doesn't give the granular data, but another reason you can't register on your own hardware is that they can't trust your device (since fake registrations could create free money for one person). The 'how it works' page here https://worldcoin.org/how-it-works specifies that they are doing integrity checking of the orb software.

The Aadhaar project faced the same issue, and learnt this lesson over a decade (ending up with trusted hardware, on-device signing, and rooting out untrusted operators).

Unfortunately, Worldcoin plans to deploy it in untrusted environments, which sounds like the biggest "hack me for free money" sign ever.

Few realize this.

It's more than "not open to general use" -- Apple's ID solutions are explicitly designed so that they only work for on-device authentication, and cannot be used to, as a random example that I'm sure no one will ever try to actually build, create a centralized database of biometric signatures for every individual on earth.

From what I understand, the only information an iOS app gets from face ID is a "success" or "failure", no actual information about your face.

Specialized non-consumer accessible hardware is a huge stumbling block for these types of solutions.