Unity 3D Terrain tools allow you to make smoothed surfaces full of trees and grasses to allow your characters to move around in. There is default terrain tools in unity, some older tutorials will tell you to download asset packs but this is no longer necessary.
How to make Terrain in Unity! - Brackeys
MAKING TERRAINS in Unity 2018 Beginner's Guide
The terrain tool allows you to paint trees onto your terrain. In order to do this you are going to have to make some tree prefabs to paint.
https://docs.unity3d.com/Manual/class-Tree.html
Please note that the assets used in the default tutorial have been discontinued from base unity so you will need to find/make some of your own.
Otherwise you can find the default textures in this bundle:
DO NOT IMPORT THE WHOLE BUNDLE! Not only is it huge but it will throw back hundreds of errors due to legacy code. Instead just import the bits you need.
You may also be interested to try this plugin that allows your trees to be randomly generated so they don't all look the same when you paint them in.
https://assetstore.unity.com/packages/tools/utilities/tree-randomizer-59228