My wife, a surgical technician, hasn't done surgeries like this before but has mentioned how for other procedures (on the face, perhaps) that they fold back the skin and then 'flop it back on'. It is crazy when she talks about stuff she's seen/helped do.
I like to picture surgery to be very similar to working on a car, except the pipes, wires, manifolds, etc are made of organic stuff.
The blood vessels all run within the scalp, which is part of what helps it be loose. Some run up the side and some come out next to the eyes, and nothing has to go through the top half of the skull.