The basic idea is this. You know Canada has certain demographics. A part of the population is male and female, of a certain age, of a certain income, etc. You ask everyone in your survey those questions, as well as the things you want to understand, and then you can use some techniques to match your survey to the rest of the country. From here, then you can estimate what the entire country thinks about given just the survey.

As one can imagine, the process of matching called undersampling/oversampling matters a lot in practice.