Here is how the return Asia-US-YYZ works

You arrive at your US port of entry, last time for you it was ORD and this time DTW.

You enter US immigration. DTW have the kiosk terminal which works for Canadian passports as well. You scan passport, answer question (same questions as the blue form), the terminal takes a photo of you, you are then issued a receipt which you take to a CBP officer and you are then clear to enter US.

At ICN when you check you bags, you should get a purple tag onto your bag which designate it as international transit baggage. This eliminates having to claim you bag, go through US customs with them and then re-check them. You will see them put the purple tag on and staple receipt for the bags as well as part of the purple tag onto your DTW YYZ boarding pass. This is a new system that have just been implemented at ATL and DTW that I have used as of late 2013 for internation arrival to international departure transit baggage.

After you exit customs, you will be subjected to a TSA flight security check, after that you will be in the main terminal where the A gates are. DTW YYZ flights are usually at the C gates. The tunnel to C gates is not far from where you enter the terminal.