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.