Can plants be used as building materials? could you give me some examples from past and current ones?

Not the plants but the soul could be made into mud in both past and present. In past, most houses(not in the US) but in many countries were made up of soil. But know in present, soil or even mud is used to put it in the concrete to make the building strong in many countries like India. Good luck:)

Yes, plants can be used for building materials.

Wood of many strong trees was used to build temperorary houses. Wooden toys were also famous earlier.

Today, Timber wood is used for making ships. Wood is also used for making furniture.