Then, those pieces are fed to the programmed yeasts, which transform them into a variety of new food ingredients, including proteins, fats and acids
PET consists of repeating C10H8O4 monomers. Glucose is C6H12O6 and starch is a natural polymer containing it. It's not surprising that reactions can turn one into the other, along with other compounds containing hydrogen, carbon, and oxygen.
Vegetables grown in shit, garbage, and pesticides are fine as long as the levels of harmful residues are low enough by the time they reach people's mouths. Not appealing, but also not that "dystopian"