A friend of mine recently flew to the United States via Air Canada.
He says he cleared customs in Canada and the plane arrived at a domestic terminal.
Why is that?
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What your friend experienced is something called Preclearance. This is a program where you pass through US immigration and customs at your departure airport, and arrive into the US as (basically) a domestic passenger.
Preclearance originally started as a way to allow flights from select Canadian airports to land in US airports that don't have immigration/customs facilities - as the passengers passed through US immigration/customs before departure the flight could land at any US airport, regardless of facilities.
It has since expanded to cover most flights from Canada, as well as some flights from select other international locations (all of which are listed on the page linked above).
because of the Canada–U.S. Agreement on Air Transport Preclearance you can clear customs at select airports at departure