I have Papri/Acrolink AgCu 8-core cable in my hands... it`s very transparent and "crytal clear" I would say... and it`s teflon:
https://www.aliexpress.com/item/Headphone-Wire/32767554889.html?spm=a2g0s.9042311.0.0.27424c4dpLFpo9
Also, ask hakuzen about PVC cable DIY wires... there are pretty little choices and most of them are much pricier than other type material coated wires.
Thus, I conclude that most those cheap chi-fi cables actually use other than soft PVC... PE,TPE,Teflon, silicone etc. .
I have some different cables from KZ, MPS, Tennmak and none of them seem to have such soft PVC coating as IT01 cable.
Maybe there is different type and softness PVC, idk. Anyway, for DIY cables, it is hard to find soft PVC coated cables with good material and core-count... with reasonable price.