You spend the 14 days in quarantine either way - land or air. And it's not 'government quarantine' for most people - it's just a hotel room, airbnb or wherever your approved plan says you're going to quarantine.
"As has been the case for months, all travellers will have their quarantine plans reviewed by a government official and, if not suitable, will be required to quarantine in a federal quarantine facility."[1]
Canadian citizens and permanent residents always have a right to enter. Presumably if you come to a land border without a negative test, you won't be trusted to self-quarantine properly and will be whisked off into a federal facility.
I think it's highly dependent. I came to Canada a few months back, completed the Federal and Provincial quarantine plans. All I got at immigration was a "be careful" and 2 calls from the province asking if everything was ok.
I've heard that that is generally the case. I was merely pointing out that if you show up at a border crossing without a negative test despite it being required, you likely won't be trusted to "be careful".
