How airlines select new Captains...? How does an airline select Captains for a new but different airplanes (ie, Boeing vs Airbus) that they just added to their fleet?

As an example, Delta has purchased many new Airbus A220. Since it is a new airplane and no one has any experience with it, do the pilots train the same for this new airplane and then based on their seniority one becomes Captain and the other a FO...or...they train for their respective position from the start?
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).