Where the plastic come from and how do they sort it? ...
Answers: Plastic is a carbon polymer. There are many types of plastics and appropriately, there are oodles ways to synthesize it. If you're really interested, there is an entire enclosed space of study called polymer science, which should be available at most university.
Plastic is petroleum based, along beside a bunch of other types of chemicals for varied types of plastics have different purposes and attributes. Most plastic is heat molded to the form it is needed.