At YYZ (Toronto) you might be able to use the intl arrival US onward departure check where there is a CBSA passenger check instead of entering Canada through a formal immigration and custom process. It is a passport check and then you are at US CBP preclear area for US departure where you go through US CBP immigration and customs. You will be in the departure area with limited amusement; maybe a simple lounge if you qualify.

Many US passengers transit through YYZ to take advantage of certain flights, their timing and cost.

As for Montreal I am not familiar with how YUL intl arrival US onward departure passengers are processed.

Since your flight to NYC is the next morning you probably will be force to enter Canada to spend the night because US CBP does not clear you into the US departure area for next day's flights.