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.
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.
https://www.canada.ca/en/transport-canada/news/2020/12/pre-d...