AFAIK inbound WS flights transiting to US will have the bags tagged to US final destination. At YYZ these are sent to US CBP.
You deplane and head to US CATSA preclear. There is a priority lane for NEXUS. As s you are on Canadian soil TSA Pre does not appy. After security your BP is scanned, if your bags has been photo'ed you will head to US CBP. If not, you are sent to a waiting room and a monitor will let you know that your bag has been photo'ed and then you can head to US CBP.
At US Transborder departure lounge options are limited. Look at the PP lounge before commiting the cost of the visit (if it applies to you)