It's funny because my first thought when I read this was, 'You're asking the wrong person!!' Lol. I can only imagine that they're like...hill-like pockets of fat that sit on the ribcage. As for armpits, it seems that when an arm rises, then the chest that's by it raises a bit too. I personally like to draw a bit of musculature between the female chest and shoulders that almost resembles the way it works for males, which, for all I know, may even be technically accurate: I imagine or draw a line running from under the chest up towards the arm, to turn into that area that forms the armpit and continues on around the shoulder.