They have lots of information on how they do their surveys.
The Angus Reid Institute conducts its polling entirely through online research— surveys or studies conducted on the Internet via a personal computer, tablet or smartphone. We have made the decision to conduct our research solely through online means because traditional telephone based approaches...
angusreid.org
Generally, a survey has a target population that they are trying to draw insights on. That target population will be defined in the survey parameters - could be all adults, just teens, etc, etc.
It looks like Angus Reid aims to target all adult Canadians in their surveys.
Random surveys work by choosing a random subset of the population, and using that small sample to make assessments of the views of the whole target population. By choosing the sample randomly, researchers can get very accurate predictions from a relatively small sample. They will also take care to make sure the sample is balanced across known characteristics like income, etc, so the random sample has a better chance of being representative.
All of this to say that over 70% of canadians wanted the protests over with.