Plastic is probably going to be the first item in Forager that leaves you a little stumped. How is plastic made in the real world? Plastic is an item that you must craft in this game. You will not be able to find it out in the wilderness.
How to find plastic
To get plastic you will need to build a factory. Plastic is an item that you can produce in this building by providing it with glass bottles of oil. The oil will be converted into plastic and you can then use this to craft many other items that are also available at the factory.
Crafting a factory requires gems, as do many materials used in a factory. If you are having trouble finding Gems, follow this guide for getting lots of gems in Forager.