Actually part of the women are pornstars, usually less famous or aspiring ones(i spot a fair share of them), those are the ones that usually end up sucking and getting the facial. The rest may be sex workers (dancers, escorts etc) or simply girls that are very open to these kind of things. Sometimes one of the girls paid to be just a "clapper" and party girl can end up sucking if she feel like it, but its clear from the start as said above that the ones who will end up making the most of the job are selected. Thats why some girls do absolutely NOTHING in those scenes, and sometimes the guys aproach them and there like "nono"
EVERYTHING in porn is staged. The only thing that is not may be some kind of hidden cams on pornhub or other tubes websites, in wich case those videos could end up on big lawsuits You need an official "signed release" to publish porn. Not just a "oh there is a cam there, its fine if we film right?" type of thing.