For one, women are that flexible.
For two, it is also a webcam, and that is the most common webcam angle.
Either they have a very elaborate magician style couch with a hole cut into it, or it is real.
You can watch full movement of the body on two of them. The pussy doesn't change, so they aren't switching bodies.
I don't know why you find this so hard to believe actually, there are plenty of very flexible women.