The drop is provided to class on day 1 and you get to select on day 2 by seniority... Class drops are pre determined by vacancies on previous pilot vacancy bids only.

Once you get a awarded a base/fleet on day 2 you are then at the mercy of future vacancy bids announced by company and can only be awarded if your new seniority can hold the base and fleet to fill a vacancy. In other words "who knows".

Important to note you are seat locked for 2 years once you start training except if you bid a larger aircraft category or bid to captain (not option). If you desire SFO best bet IMO is 737 based on recent bids and fleet planning announcements.