Many airports have an international transit zone, which is inside the legislation of the country, but in a legal fiction travelers haven't immigrated, yet. The

Many airports have an international transit zone, which is inside the legislation of the country, but in a legal fiction travelers haven't immigrated, yet. The only one having to go through border check there are travelers connections to domestic flights (in EU inner-Schengen flights are domestic) or for leaving the airport. U.S. airports generally don't do that. Even when transferring from a flight between Europe and South America they request you to go through immigration and customs.