Typically, heavy maintenance is one that will be contracted out. Line maintenance, or things that come up flight to flight, will often be done by the maintenance staff of the newly created carriers.
This is a complex area. One of the big issues is inventories of spare parts. It's very expensive for an airline like WestJet or Air Canada, given the hundreds of points they fly to, to keep a complete set of spare parts. That's one of the reasons why this industry has evolved. We're seeing more and more contracting out, even these days, of some of the line maintenance, simply because if you contract out all the line maintenance of one airport for a particular aircraft type you probably will have the needed trained staff as well as the spare parts.