what are the different types of self twist yarn?

Question

the different types of self twist yarn and its properties

Answer 1

Never heard of "self twist" yarn.
From all my experience, yarn is spun from strands of silk, rayon, polyester, polypropelene, nylon, wool, cotton, etc.
Then they are plyed together, using various numbers of those fibers. The "plyed yarn" is then twisted, 2 1/2 or more times, then heat set, some by hot water, some by controlled heat.
The yarn can then be used as a single ply or plyed together with one or more plys, creating two ply, three ply yarn, etc.. The most plys I've seen are seven, sometimes used in carpet.
Most carpet may contain 2,3, or 4 plys of yarn.