As for the sleeves, they are all bought from manufacturers. The front and back are in the same direction, and the sleeves are covered with silk.
What you don't understand is how to make a set of silk, that is, to use a special straight thread set of silk machines, and there are molds on the machine to clamp the steel bars on the machine. There are several kinds, such as diameter change. The rebar threading machine is used for threading.