from what materials is a plastic made from?

there are many, many types of plastics. Most are sythetics. But one natural one that I can think of is rubber. But the resins from the plant still need to be polymerized by a process called vulcanization. This is one but there are many

Synthetics are made from monomers. Monomers are the single units of polymers. Like ethylen is an example monomer, heat under intense pressure and temperature and it will polymerize by free radical chemistry.

Nearly all plastics orginate from crude oil.

It ias heated and at different stages of the heating process, different constituents that make up crude oil can be siphoned off and processed into useable plastic.

x