hi, squirter here!
the porn girls usually are peeing. sorry to demystify that for you. i generally don't "believe" them. there are a few that i'm sure are completely legit, but i'd say the majority have had a few 12-pks of water (or whatever) and are letting go.
i'd recommend axel braun's videos. squirting is NOT genetic (sorry, tripod), and it has to do with stimulating the glands that are around the urethra. basically, these 'fill up' when a woman is aroused and you can coax the fluid out. some people squirt with clitoral stimulation, some with anal stimulation, and others with gspot stimulation.
for me, it's all gspot. i discovered with the help of a boyfriend while i was separated that i could squirt. he had seriously talented fingers, and it became a party trick ... putting out candles, etc. others could do it as well, and one guy with an enormous head and girth made me cover most of his bed. i think i can squirt while using my hitachi magic wand on my clitoris, but i have not verified this with The Husband. he, unfortunately, cannot make me squirt, no matter how hard he tries. we have several gspot stimulators, and all he's gotten is a fine mist. ah, well. memories...