U.S. immigration is done outside the U.S. at 16 places around the world. When you arrive at the airport you go through US customs and immigration before getting on your flight.
That’s great for non-US citizens because the lines are usually shorter and they can make quicker connections on arrival. That’s great for the US, which is requiring that foreign governments or airports pick up the cost of immigration processing. And it’s not so good for US citizens who may face delays they wouldn’t otherwise with standard immigration.
And they may be expanding immigration preclearance to 11 new airports.