Lately it has been SLC, DEN, ORD, FAT, COS.

Whatever openings the company has that have not been filled by existing pilots arre offered to the new-hire class. The most senior person in class picks first, so you get whatever is left when your turn comes around.

Seniority is determined by last 4 of SSN (ie 0001 = good, 9999 = bad). Any new hires from ASA automatically have top seniority, there are usually a few of those.

If you don't get what you want initially, you can rebid, and will likely get it soon (except the super-senior CA bases).