I'll have an international to US connection in Montréal with Air Canada, and after looking at various websites (including Air Canada, Montréal airport, and others), the process is still a bit unclear, especially with regards to the ArriveCAN app and canadian customs.

The airport website says that "If your airline offers the option of automatically transferring your luggage and you have a boarding pass for your connecting flight", you do not go through canadian customs and just go straight to US Customs preclearance. But it doesn't say which airlines offer this option. I would assume Air Canada would be one of these, but want to be sure.

The Air Canada site says "Provide your baggage information to the Baggage validation attendant then proceed down the escalator to the security check point" and then to a "customs declaration" before US Customs. But it doesn't specify if the "security check point" and "customs declaration" is Canada customs or something different.

So for those of you who have done this before, what is the process? This makes a difference because if you never leave the sterile area of the airport, you don't need to fill out the ArriveCAN app as you won't be entering Canada. I guess worst case scenario if you find out you need to go through Canadian customs upon arrival, can you fill out the ArriveCAN app while you're in line at customs? Or does it have to be filled out beforehand?