100% the person who leaves the truck is a man
The person driving the victims' car who briefly steps out and leans up, IMO could easily be female. Pale skin, long very dark hair, small build and agile. I think they're passing the gun or something else they don't want to touch out of the victims' vehicle.
So, IMO someone in the truck passes the heavy dude out a rolled towel / cloth, he gives the driver the towel for them to put something in and they hand it back with something inside. Unsure why they would slightly exit the vehicle, to what purpose. It's not a clean up job IMO, it's handing the murder weapon out probably.