You will only get assigned where vacancies exist at the moment they assign bases. So if there were 3 spots in say CVG and 15 spots in NYC, and you were the top bidding in your class you would have first shot at CVG over the rest of your class.
You won’t “pick” your base as much as rank them and then you get given whatever is is available. It will 99% be NYC or CVG out of training. If you’re lucky you may be able to get DTW out of training or soon after. MSP and ATL are going to be a while.