During regular times you don't enter Canada and enter US pre-clearance down a sidehall. I can't speak for covid times. When is your flight - you may have trouble boarding as Americans aren't able to enter Canada at the moment so you may not be able to depending what your Greece gate agent says....

Usually the YYZ airport has a connections guide but even they have it disabled saying to ask them directly... https://www.torontopearson.com/en/connections