The season you first join, the leave bidding process will have taken place long before, so you’ll be outside of the normal bidding process for it. I am not entirely sure how it works, but my guess is it is just allocated. You might be able to express some preference a bit like the training line preferences mentioned above.
This Is correct, but if you have specific requirements for leave, tell BA and they will try and accommodate you ("i need that week off to be best man at my brothers wedding" will usually get a better response than "i just fancied it as the flights were cheap"). When I joined I asked and crewing were kind enough to give me 10 days that overlapped a big family holiday over 1/2 term, which I probably wasn't entitled to but was very grateful for!
On a more general note, that's the best & worst thing about BA, if you find the right person in the right department, you can usually sort out anything, 99% of people just want to help & it's a big enough conpany that they can do it. But getting to that right person can be nigh on impossible at times, which makes it incredibly frustrating.