What kind of “knitted yarn” are you talking about? Both wool and polyester are yarns that can be knitted to make sweaters. Polyester is a lousy yarn for knitting: the fabric knitted from it is not warm in cold weather, and in warmer weather, it quickly becomes sweaty and damp. Like its synthetic counterparts, it is dirt cheap, which is why virtually every cheap sweater you see in the shops is made of some kind of man-made fibre.
Lots of things can be made into knitting yarns. Plastic shopping bags, for example. Paper. Stainless steel. Silver. Torn-up clothing. Milk. Eyeballs. Some make warm sweaters, some don’t, some aren’t meant to.
I’d suggest you join Ravelry, and spend 6 months or so in the Yarns section, studying all the different fibres, constructions, weights, blends, and drafting methods and how these influence each yarn’s qualities, dyeability, fulling/growth, draping, etc., etc. Spend some time learning from others on the Yarn and Fibre Forum, even ask a few questions. Discover and follow Rav yarnies like Clara Parkes. Take in a handspinning class. KNIT SOMETHING.