Is it possible to transfer courses from a Canadian college to McGill university?

You need to talk to your academic advisor. They can look at course matriculation from your school to McGill. It will help you alot once they tell you.

You don't transfer to McGill. You can apply for admission to McGill and, if admitted, you can ask to transfer your Langara credits. If they are university transfer credits (i.e. if they would transfer to, say, UBC) then almost certainly McGill will accept them. It will however be entirely at McGill's discretion what credit to give them, i.e. transfer credit to specific McGill courses or unassigned credit. And no McGill will not tell you ahead of time; people have to much to do dealing with real transfer credit requests to deal as well with hypothetical requests so you just have to take your chances.

Yes, it's possible. I know someone who transfer ed from Niagara College to Brock University

The person who explained that you apply for admission and then apply to transfer credits has it right. The term "college" in the USA includes what Canadians would call degree-granting universities. In Canada, people usually distinguish between colleges and universities. If you really want to go to McGill, do talk to the admissions people and see if you can get credit for any courses you've taken.