How does oil become a plastic?

Most modern plastics are derived from petroleum.  Taken from the ground in the form of crude oil.  It is sent to refineries.
 
​ Here the oil is heated and seperated into various types of hydrocarbons.

​The petroleum seperates.   A portion of these chemicals, monomers are used are combined into long chain molecules called polymers.  From these, various types of plastics are produced. ​ 
It is a complicated processes that requires lots of heat and  uses toxic chemicals..