You'll go through a CATSA security screening but not Canadian Immigration. This is assuming you're booked through on the same airline/airline alliance and will remain in your terminal. If your booking requires you to switch terminals, then you will need to go through Canadian Immigration as there is no airside terminal transfer. At Toronto (with a few exceptions), Star Alliance is in Terminal 1 and everyone else is in Terminal 3.

Thanks. It'll all be 1 ticket on Air Canada so it should all be in Terminal 1 🤞

This is not correct.
All PAX arriving into Canada go through Canadian Immigration, but it is a slightly quicker check for people departing to the US or other international destinations. At US pre-clearance you will have security screening AND US CBP processing to enter the US. More here:
https://www.aircanada.com/us/en/aco/.../int-us.html#/
Link could be clearer:
1. there are no longer any Canada Customs cards to fill out. You will answer the screening questions on a kiosk.
2. the page makes no mention of security re-screening or US CBP clearance, but by ollowing the signs for the F gates showing the US flag, you can't avoid the pre-clearance centre.
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13F

Yes, AND you need to be admissible into Canada if you are connecting Intl-Intl or USA-intl because you can just walk out of the intl concourse without immigration inspection. You don’t have the same setup intl-USA or USA-USA because you can’t exit to the street without going through formal immigration.

If you’re found inadmissible into Canada during the transfer, it’s a big deal because you’ll need to be formally deported with an immigration agent doing all kinds of paperwork and then handing you over to the departing airline and waiting to make sure you board and the flight leaves the gate.

OP was flying Punta Cana - Detroit. Afaik they normally only actually see US CBP. They wouldn't normally have to go to an interview with a Canadian immigration officer. Are they going through "Canadian Immigration" in some back office procedures sense? I'm sure Canadian immigration gets passenger manifests and could pull them aside but I don't know that that's likely.

They do not go through formal CBSA processing but they are subjected to a travel document check by a CBSA officer to head from arrival to US (or ITNL) departure.

I guess I don't normally travel this late (thanks Jazz pilot shortage and their rolling delays.)

Just realized the almost EVERYTHING closes after 23:00

Arriving from US to Dom connection, transfer area is closed, got kicked out to the land side after clearing CBSA.

Verified travellers lane was closing right in front of me.
The regular security lines wraps all the way to the check in desk area full of Air India and BR passengers, it was an absolute chaos.