Officially, if you try to enter Canada with a positive pre-departure test, you may be subject to a fine of up to $5000, but as a Canadian citizen you can't be denied entry. Unofficially, people are reporting that at land crossings, fines are not routinely imposed and those with positive results are just being sent home to quarantine, as long as there is an appropriate quarantine plan in place.
I've only flown internationally once since border restrictions were put in place, and on that occasion the Air Canada check-in agent insisted I had to have ArriveCan approval before I would be allowed to board the plane. So in that case, if someone didn't have a negative result they wouldn't be allowed to travel, and would have to quarantine in place, according to whatever the local rules are, until either a negative test is obtained or 10 days have passed after a positive PCR test. But that was from The Bahamas, last summer, so YMMV.
There is also still random arrival testing. If you are tested, you don't have to quarantine while waiting for results, but of course if your result is positive you would be expected to quarantine (at home) at that point.
All of this assumes you are vaccinated, of course. If not, I believe you are subject to quarantine on arrival in any case.
Also, re ArriveCan, the lab won't upload anything directly. They will provide you with documented results (usually by email), and YOU will upload that into your ArriveCan account.