FAs bid for position. I have a junior FA friend who bids for FL and often gets it because senior FAs don't want it.
I don't know the specifics at all, but there are preset responsibilities for each FA position (FA-A, B, C, etc.) For example, FA B might be assigned to the aisle during boarding on 319/320s. Are there any FAs here to confirm or correct this? I only learned of this when I asked a FA why she wouldn't help with the bins and she said "that's the B line's job" or something like that.