The PTFE itself is really an inert polymer that is harmless to the water flowing through. It will not get into your water or break down through decay, those were considered superior properties. It takes a lot more heat to decompose PTFE than it does regular plastics, but once that's happening the vapors are a lot more toxic. But these are not the kind of plumbing fittings for welding or soldering.
Chemically, ethylene is the gaseous hydrocarbon monomer that is polymerized to make the common polyethylene plastic, like so many milk jugs and other items needing a cheap plastic are made of.
Tetrafluoroethylene (TFE) is the fluorocarbon monomer which is the fluorinated analog of the common ethylene. It's really rough and wants to spontaneously react with itself (or any number of other things) to form a bit of polytetrafluoroethylene (PTFE) polymer with somewhat explosive force under not-fully-predictable conditions. So nobody really wants to handle TFE.
Instead extensive research at places like 3M and DuPont came up with a number of different alternative starting molecules that were less self-reactive but could still be coerced into forming PTFE as a final product.
The PTFE was intended be a forever plastic like no other, but a variety of fluorinated starting materials and byproducts were also obtained some of which had interesting properties themselves. For instance some acidic PFAS compounds are not very inert on their own, one end of the molecule will firmly bond to some natural products like canvas or leather, which largely neutralizes their further reactivity. So after that those molecules aren't going anywhere until degradation of the substrate occurs. And the other end of the molcules possess the perfluorinated inert feature, forming an invisible coating which repels water like people had never seen.
So it's all this other stuff, and the final degradation products (of those PFAS that have a decomposition pathway), that ends up in the environent eventually, many with a physical propery of self-dispersal. OTOH the PTFE is a solid plastic and just sits there permanently inert, no decomposition pathway, with little tendency for further environmental spread unless incinerated or microscopically pulverized, which disperses it as a byproduct gas, or very tough microplastic, respectively.