> If you go through customs as a US citizen, you don't even need to hand over your passport: they just scan your face now.

It's always amazed me how well this works when they are scanning you with the same 2015-era cheap logitech camera I have.

They already know you’re coming in and on which flight so that really narrows it down. I didn’t think faces were that unique and that they were matching on the subset of people they were expecting at that time. Perhaps the technology is better than I thought.

The "magic" is that they have it all joined with the database that tells them who crossed the other way recently, the flight database or the cruise ship database so it's not searching through millions of passport photos every time. They have a pretty good idea of the search space.

US requires all passenger manifests for flights are forwarded to USCBP 48 hours or more before the flight. This list is run against several databases to identify persons of interest.

Then after the flight takes off, an updated list is sent again to USCBP.

So when you walk up to USCBP, they already have a list of people they expect to see within an hour of the flight landing. The match is much easier at that point.

It all falls apart at land or sea border crossings. Requirements are quite different and travel outside continental US but with zone like the Caribbean are treated differently than travel outside that zone.

As such your immigration record can get messed up if you say exit via land border to Canada as there is no exit record to match up with entry. Many people have gotten email about overstays because of this.