Originally Posted by
tentseller
IIRC, all International arrivals clear CBSA at the first port of entry YYZ, and then you are in the terminal landside. You need to clear CATSA to get airside. The only exception is the transit-to-the-US bypass, where you undergo a document check by CBSA and are sent to US pre-clearance.
The only full airside transit is domestic-to-domestic.
That changed several years ago. You do clear customs and immigration and stay airside for your domestic connecting flight unless notified otherwise. I do it all the time as I am based in YQB.