I am a USA citizen but am going to be flying out of Canada to go to Scotland. When arriving at the Toronto Airport, will I be going through customs or immigration before receiving my boarding passes? I need to know because our airline was very adamant that we are checked in 3 hours prior to our flight.
If you are flying to Toronto from the USA, you can use the Connecting Info form on the Toronto Airport website and it will detail exactly what you need to do. Just enter where you are coming from, which airline and where you are going to and the airline you are flying. https://www.torontopearson.com/Connecting.aspx
If you are driving to Toronto from the states you will deal with border formalities wherever you cross into Canada.