Originally Posted by jamespvg
EDIT: I'm also fairly sure it's not as simple as it being in the same terminal. US flights are separate in Canada and DHS does immigration pre-clearance at Canadian airports (they arrive at the domestic terminals of US airports). Certainly our arriving and departing flights were in completely different sections of the international terminal.
The International Terminal is a single, large terminal, though the check-in areas for US and International flights are in separate halls. The gates feed into both the US and Canada immigration halls, so for example you can do your trip in reverse without having to go through Canada Customs (instead y
you just pre-clear US immigration and customs at YVR).
Signposting is pretty clear (International connection routes are indicated by a globe logo IIRC, US connections follow the US flag signs, Canada arrivals follow the Canadian flag signs).
I would also guess that different solutions will be used at different times of day, but the bottom line is that if AS can give you your BA boarding pass, then you do not need to exit all the way to the check in hall.