All files on the phone will be checked against a hash database before being uploaded to iCloud. A cryptographic safety voucher is created for each photo as the

All files on the phone will be checked against a hash database before being uploaded to iCloud. A cryptographic safety voucher is created for each photo as they're uploaded. The iPhone doesn't know wether or not anything matched. Nothing happens unless the user reaches a threshold of 30 CSAM images that have been uploaded to iCloud Photos.