My friends connected through YYZ on an itinerary PHL-YYZ-AUH, AA/EY. They had to collect their bags at T3 and go through customs before dropping the bags for EY. There were told this is for all flights through Canada from US-INTL. YYZ website says that, but I don’t see the same for connection on *A through T1. Looks like there is no customs there. Why the difference ?

do these rules apply to all Canadian airports when connecting from US to another INTL destinations ?
I am connecting through YUL on a similar itinerary and wondering if I need to clear customs at YUL just like YYZ