How are these parachutists restrained in this type of aircraft? Do they have seats fitted, restrained in any other way,
Not sure about other types, but when I was meatbombing back in the day the skydivers had straps made from seatbealt material that they attached to their harnesses. Although getting them to where them was often a challenge.
Skydiving aircraft dont have seats fitted (and no, its not a weight or room saving idea, this comes at the expense of other things, like radios etc). It is to reduce objects that ripcords and other things can get caught on and release the canopy inside the cabin inflight.