Basically, once you pass the interview, they may tell you what bases will be offered at your class date. On the first day of training everyone (in order of age-oldest to youngest) gets to pick a base from the list that are open. After that, in class seniority order by base, you pick the eqp you want. if you dont get what you want the first time, then just wait for the next vacancy to be posted and put in for what you do want. Once the next class starts you will no longer be the most junior guys obviously and can move around more easily.
heres what equipment flies from where, in case you interviewees and recent new hires are wondering:

DFW: CRJ, EMJ, ATR
LGA/JFK: EMJ
ORD: CRJ, EMJ
LAX: EMJ
MIA/SJU: ATR

at this time the most junior pilot in each status is fairly even except for LAX. I cant imagine not having at least 3-4 bases to choose up to a certain point.
Another thing about that. They may also say "we have 6 spots open in mia, 8 in sju, 11 in dfw, 15 in ord. Or whatever. whoever is the oldest in class gets first pick and if your last, well, u get what the rest didnt want.