They may just ask you the serial number of your device during purchase and ship you a part that's already programmed to it, so it's a drop-in replacement and will "just work".
They could also provide a software tool to do so or even have the target's device firmware re-link the part at first boot (potentially checking the serial number of the source device to prevent iCloud-locked phones being used for parts).
They could also provide a software tool to do so or even have the target's device firmware re-link the part at first boot (potentially checking the serial number of the source device to prevent iCloud-locked phones being used for parts).