When flying from a Schengen area country to the US, border patrol on the departing side is entirely managed by that country. Often e.g. United-operated flights

When flying from a Schengen area country to the US, border patrol on the departing side is entirely managed by that country. Often e.g. United-operated flights from larger European airports will have their own passport check, but that always happens after you pass through the departing country's passport control. At least, this is my personal experience flying between Germany and the US ~once a month. I have been to a few airports where you explicitly clear US customs at the departing airport before leaving for the US — Vancouver, BC and Dublin Shannon come to mind. If you've overstayed your automatic 90-day Schengen area travel visa, a common hack for backpackers and such is to intentionally leave the Schengen area from someplace like France that's generally less strict about counting exact days than many other EU countries. To be fair, I don't have much experience flying from the Schengen area to non-Schengen area non-US countries. It's totally possible this is all rigamarole that the US government pressures other countries to do.