I am coming from the US to Montreal airport catching another flight to a CDN city. Where will my customs be?

It depends on what your final destination is. For a major city like Toronto, they have special flights just for US connections, so you stay within the US area of the airport and only go through customs in Toronto. (That's not a rule, just a possibility, you still might have to go through customs in Montreal). For smaller destinations, you will definitely have to go through customs in Montreal and then move to the domestic wing of the airport.