It's a pure lottery system. If you are lucky you can be assigned to a very popular base as your first one. if you are unfortunate and sent to a place you didn't know existed before, you can wait 2,3,4+ years before moving to your desired place. At the moment all the transfer lists are full of people who joined in 2023 and 2024. Sometimes more than 200 FO waiting in the queue, to give you an idea...
If you miss your chance as a cadet, the hardest bases to get are probably BGY, VLC, ALC, MRS, CIA, OPO, FAO... actually it would be faster to name the easy ones to get, not sure which ones