You only have control of the muscles you use for "bearing down". Either it's your vaginal muscles or your rectal muscles, but what comes out and where it comes from, you have no control.
During labour, there are involuntary contractions from the impending delivery. When you "bear down", you are consciously forcing your vaginal muscles to assist in pushing the fetus out. Similarly when you are taking a shit, you have involuntary contractions of the impending feces expulsion. When you "bear down", you are consciously forcing your rectal muscles to push out and assist. In both cases, what comes out and where it comes from you have no control of.
During squirting, the woman irritates her g-spot or urethral sponge to encourage orgasm. Eventually there are involuntary contractions. The difference with squirting as opposed to a normal orgasm is that she is trying to encourage the expulsion of liquids inside her vagina, so she "bears down", to cause her vaginal muscles to assist in the expulsion of such liquids. What those liquids are, and where they come from, she has no control. The Skeenes gland is irritated/coaxed to release its liquids, but that would only account for up to 50ml of it. The rest of the liquid is so voluminous it can only come from an organ that holds that much liquid. There is no other liquid other than blood that is so voluminous, and that liquid has to be what is stored in the bladder.