Europe to US via Canada (YYZ or YUL) question

I can answer one question here- at YUL/Montreal, you go through US immigration before boarding the plane for the US.

Cat is correct about pre-clearance.
Between the two airports I would choose Montreal (YUL) over Pearson Toronto as it is smaller and not as busy.

Flew last year through YYZ from SFO to Lisbon.

• Average time for layover: would have minimum 2 hours. Our flight from SFO was over two hour late and we needed the 3 hours layover we had in Toronto.
• US immigration is processed in Toronto
• Didn’t check luggage, so don’t know if you have to retrieve
• Personally, I have had layovers in both. I choose the one with the most options to fly to my destination in case there is a delay and that is typically Toronto.

Here's the connection guide for YYZ.

https://www.torontopearson.com/en/connections

Are you flying Air Canada? Here's their connections guide.

https://www.aircanada.com/ca/en/aco/home/book/why-book-with-us.html#/

We fly from YFC to the US via Montreal or Toronto several times a year. Our bags are always checked through (and that holds true US to Canada or returning from Canada). You will go through Customs at Montreal or Toronto. Montreal is a bit easier as it is smaller but neither is too bad. You will have to go through security again. This is handled by Canadian Security so no TSA Pre-Check or anything like that. Your flight will arrive in the US as a domestic flight so no further Customs stop. In the last year or so it seems that passenger traffic to the US from Canada is down and wait times for Customs has been short. My last trip Security took about 15 minutes and when I got to the passport check there was only one US Customs agent but I was still through in a few minutes.

Well I opted for:
LIS > YUL > SFO
First leg LIS to YUL on Air Canada
Second Leg YUL to SFO on United (it’s a code share)
The layover in YUL is almost 4 hours, so plenty of time.
From what I understood from your comments I don’t need to retrieve the checked luggage at YUL to go through pre-clearance. All I need is to go through US immigration (passport control) pre-clearance at YUL since the USCPB operates also in Canadian airports for US bound flights.

Well, I do know that you CAN do it in under 40 minutes if your checked bag wasn't on your flight ;)

I was rerouted through Toronto not all too long ago on my way to a funeral, but my bag was not on my flight--it stayed in Paris. So after a multi-hour delay in my departure city (climate activists gluing themselves to the runway--a new one for me) and then being automatically rebooked, running through CDG and very nearly missing the flight I had been put on, landing with a delay at YYZ and then having to wait to deplane due to an inflight medical situation, I RAN through immigration, ended up in the crowded baggage claim area, hunted down someone who looked at my tags, said my bags were not in Toronto, then said "run, baby!" as I bolted through the terminal to find my departure concourse. I went back through security, ended up sweaty, hungry, exhausted, and grieving and thus in tears in the passport line still not knowing exactly which gate was mine. Some lovely folks yelled "hey everyone, let's let this lady through," they moved me to the front of the line, I found my gate, and even had enough time to use the bathroom before boarding my flight home. Only to be delayed on my third flight by a bird strike . . . . but I got there.

But to answer your questions for YYZ:
1) I would assume 1,5-2 hours for a layover
2/3) You go through immigration and pick up your bags in YYZ and drop them off again for your connecting flight, just as you would when arriving, then connecting in the US. You then go through security again AND show your passport again. However, when landing in the US you land in a domestic terminal. Your bag should arrive in the normal baggage claim.
4) Can't say. The staff and other passengers at YYZ were so absolutely lovely, but the customs baggage claim area was a definite zoo that day. Had my bag made it to Canada, it almost certainly would have meant missing my flight to Atlanta to find it--as I said, at that point I was down to 20 minutes to departure, still had to run down the terminal, go through security and passport check, and find my gate.