IIRC the general idea is that the pin code or password is required to be entered when an update is requested, which unlocks a key pair. The public key is included in the update request, which is then sent to Apple. Apple sends back a download that is signed in a way that the firmware can verify, and Apple guarantees never to send another download in response to that exact request. This protection also relies on the secure enclave never authorizing the installation of an unsigned OS update.

> Apple sends back a download that is signed in a way that the firmware can verify, and Apple guarantees never to send another download in response to that exact request.

Interesting! Thank you for the detailed explanation.