How does this work? If they aren't uploading the picture how are they doing facial recognition? Surely they aren't running their highly proprietary recognition

How does this work? If they aren't uploading the picture how are they doing facial recognition? Surely they aren't running their highly proprietary recognition software on users' phones. Maybe they're doing preliminary zealous face-finding and uploading those potential matches for further processing? If so that means that no photo you ever take on your device is private.