Hey there! the trees and grass i think where default from unity and the rocks i got for free off the asset store here: https://assetstore.unity.com/packages/3d/props/exterior/rock-and-boulders-2-6947
I would imagine it would work in unity 2018, but you would just have to test it to be sure. I made this in unity 2019.2.9f1 but most any unity version will be compatible.