What happens to the selfie a guest uploads? Is it stored somewhere?

By design it's used for one job: matching the guest to their photos. Snaptide's explainers say the selfie is only used to identify that guest's images and is not shared with other people. Before a guest can even proceed, they have to tick a consent box — "I consent to facial recognition processing for finding my photos" — so there's an explicit step where they agree to how their face data gets used. It's about as transparent as this kind of feature gets in consumer platforms.