How airlines select new Captains...?
Qualification for being a pilot-in-command in mutli-crew aircraft (i.e. captain) is independent of type rating (i.e. qualification to fly specific aircraft type). And similarly with the multi-crew rating required to fly a multi-crew aircraft at all (i.e. as F/O).

Therefore the airline selects some pilots and sends them to appropriate training to get the type rating. And those who were captains before can be captains on the new type too.

Since much of the experience applies to any aircraft, usually they'll select the more experienced captains from other types to become captains on the new type.

If the type is widely used elsewhere, the airline will also try to hire pilots from elsewhere who already have appropriate ratings, but obviously there is not much opportunity for that in a new type like A220 (formerly known as CSeries).