I am out of the business now but only recently; things were only starting to improve for newcomers when I left, then things went crazy. It's only been a short time since the shortage began when you think of it.
So the mainline carriers are not experiencing a shortage. I think it is stating the obvious, the two main carriers get much of their feed from their feeder airlines, and have at least some relevant experience. There is also a minority who go straight to mainline in at least one of the airlines, the other one I am not sure. The job is attractive enough to attract people with some background who would otherwise choose to stay where they are.
So that promise of a shot at the big times (and eventual big paychecks) still seems to be enough to attract people to crew their airplanes. I know of one person with very little time, and I mean very little time, that was taken on by one of the feeders, spent about 3 years there, and was taken on to the mainline carrier, right seat in a widebody no less. Not everyone gets on of course, but it is still enough incentive to attract people.
Not the ideal situation I know, but it is what it is.