Customs in two countries?

You will have to go through Customs at Edinburgh Airport

Who still dinnae wear knickers ?

Whae noo. Ah dinnae like thae smile.

As you will not actually be entering Canada, nor will your baggage so there should be no need to go through Customs in Canada. You should be staying "airside" the whole time and that doesn't count as entering the country. It's kind of an international zone. In Edinburgh, definitely yes as you will be leaving the airport at that point.

If you are travelling in the same airline on a single booking, then you'll just go through flight connections in Toronto, your baggage should be transferred on to your Edinburgh flight. If you have two separate bookings, you ma need to collect your luggage and recheck it, I would ask in Dallas when you check in to be certain.

Just in Edinburgh

You lucky person you...I love Edinburgh. They have some of the best pubs along the Royal Mile leading to their castle.

Anyway to answer your question. Typically you are booked straight through to your destination. (Make sure your luggage gets booked straight through too.) Which means you'll wait in Toronto inside the international terminal and inside their secure area.

So during the layover, in TOR you can stay inside the secure area and not go through security/ customs/ passport.

But stay inside the secure area. There are typically shops and restaurants inside the secure areas so you need not leave them. If you do leave those area, then yes you'll need to do them all over again.

NOTE: While the above is all true, as a seasoned overseas traveler by air, sometimes I have been surprised. I landed in Vienna as a layover to catch the next leg to Kosovo. Both are international flights, in and out of Vienna.

I figured we'd (I was with my wife) just stay inside the secure area after arriving and wait there to take the Kosovo flight. In fact, the flight attendant assured us that our outgoing flight would be boarding at the gate right next to our arrival gate.

Did not happen that way.

For whatever reason known only to the Austrians, upon arrival we had to leave our secure area to walk all the way through the terminal to the other end to catch our outgoing flight. And that meant we went through passport and customs to leave the secure area after landing in Vienna and then...go through security, and passport again to enter a new secure area for the Kosovo flight.

In other words we had to stand in two long and very slow lines coming and going through Vienna. We almost missed our connection to Kosovo. My point, expect the unexpected in international flights, especially into and out of small airports.

You presumerably mean passport control and that will depend on if the flights are directly connecting flights/just fuel fills or if you get off the flight and go landside, if landside you will go through passport control, as far as customs/immigration, all and anyone is subject to both whether you see/speak to them or not they are certainly watching you and all and any bag you have will be scanned again if it comes off the flight whether you have the bag or not. EACH country is responsible for anyone traveling into or out of their own country for customs/immigration control

It depends on whether Britain maintains a Customs and Immigration presence at the Toronto Airport. I know the US does (for people going to the US) but I don't know about Britain. Either way you only have to do it once, not twice. Enjoy your trip!

No. Customs is for the purpose of controlling things brought into countries. I think you're probably asking about passport control. You should not have to go through that in Toronto, as long as you're just in transit in the same terminal. There are some cases where you have to change terminals, and then, since you'd be leaving the building, you would go through passport control. Your luggage is going to be checked through from Dallas to Edinburgh, so you wouldn't have it to take through customs.

You shouldn't have to go through customs in Toronto if you never leave the international gate area.