The online website, Healthline, has an article, "Everything You Need to Know About Squirting", that discusses this. The article describes the role of the Skene's glands:
"The fluid - not pure pee but a combination of urea, uric acid, and creatinine - is released by the Skene’s glands, which sit at the lower end of the urethra. When someone squirts, “it’s usually from G-spot stimulation, or clitoral and G-spot dual stimulation,” said certified sex coach Gigi Engle, author of “All The F*cking Mistakes: A Guide to Sex, Love, and Life.” The Skene’s glands, G-spot, and urethral sponge are all located in the same area. “Typically, if you stimulate one thing, you likely stimulate them all,” she explained."
For more on this, you can refer to a DVD from sex educator Tristan Taormino, "Expert Guide To The G Spot" from Vivid Entertainment's "VIVID Ed" series. Although this video is explicit, it is for sex education, not porn. This can be purchased from the Adult DVD Marketplace website.