When you book these days you are encouraged to enter your passport details into your booking. That data is transmitted to the USA so they can match it against the ESTA/visa database and I doubt it's all done in a big batch once checkin has closed. BUT, if the data is not in the PNR it can only be verified at check in. Indeed, a late application/update to an ESTA may result in people arriving at checkin not aware of a cancellation resulting in the impression that has been cancelled at check in. It's all about timing!
Having said that I doubt all the data is sent in a batch there is a message sent when check in closes asking permission for the flight to depart. I am going to suggest that having sent the passenger details "piecemeal" a final message, referencing the flight details is sent for final permission to depart.
Last edited by Hartington; 21st Jul 2021 at 21:34.