Relax!
There is no segregation of US and intl arrival at YYZ. You just look for the diversion to US connection/departure, show the CBSA agent your travel document for a quick check and you should be on your way.
To get the map and direction at the airport website use intl arrival and US departure.
US to US transit is not usually done due to it is not a fare that the American airlines are allowed to sell on one single ticket.