As stevendorechester indicated, there is a US connection stairs before CBSA inspection. While I’m not sure if CBSA is at the bottom of those stairs, I would definitely be prepare to meet Canada entry requirements as this connector is NOT open all the time and also in the event your second flight is cancelled for whatever reason and you are forced to spend the night in Canada. In any case the first flight will not let you board unless you can enter Canada to begin with.