We often use the preposition "de" for description, so, if in English you say "a plastic doll", in Spanish we say "una muñeca de plástico". Here's a lesson on this use of "de".
You can also make it more literal and say "made of plastic" by using "estar hecho / hecha de + [material]:
Esta una muñeca está hecha de plástico.
This doll is made of plastic.
Here's another lesson on this other specific topic.
I hope they help.
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