Smart Traveler: The receipt is sent back to the app. It contains a scanable code, I believe it is a QR code. Mobile passport allows for multiple travelers, so you receive a receipt for each that you have registered. After downloading the app, which you must do prior to your trip, you must enter passport info and take a "selfie" picture before registering with Customs and Border Protection.
PHeymont: The app is live at the followiing airports: ATL, ORD,DFW, FLL, MIA, JFK, EWR, MCO, SFO, and SEA. They say DEN and SJC are coming soon. Initially, it was only available at ATL, MIA and SEA. It works very well. The only problem we have encountered is that immigration people are not always prepared at the Mobile Passport station. However, the delay is minimal and due, I believe, to fewer people using the app.
I would add that we also have Global Entry. However, Mobile Passport is much faster, an interview is not required, and there is no charge.