According to the rules, they (CBSA) have to tell you at the airport and give you instructions, and then you are directed to the testing area before exiting the airport. After a few days you get an automated call (from PHAC) that asks for the tests results (apparently they don't get it directly from the testing provider, at least not if it's negative).

However, a lot of people that were randomly selected by the computer (and thus PHAC expects them to get tested), were not notified or asked to get tested at the airport, so they were surprised to get the phone calls (also an email) after a few days, that tells them they have been selected. Please note that this chain of events is in complete violation of the arrival testing rules and procedures that are published by the government, however most people accept it because it's very hard to get to a live person on the phone and explain to the PHAC that CBSA made a mistake and forgot to notify you of being selected. So in this case, you call the testing provider to ask that they send you a home test, and then you do the test in a video call, and send them the sample, and finally get the results. This usually takes 1-2 weeks.

They said they were aware that this happens, so I hope that when (and if) the arrival testing resumes, there will be a way to resolve mistakes like that, because it's quite stressful to deal with this for two weeks after arrival. My general advice, is to specifically ask the CBSA agent that examines your passport, if you are selected or not. If you are selected, it's best to just do it at the airport IMO.

BTW, I'm not recommending this, but I actually asked to get a confirmation that I wasn't selected in writing but the CBSA agent refused. But at least I got a verbal confirmation of not being selected. So that hopefully reduces the chance of mistakes, and being unknowingly selected.