Do you place each individual tree on the terrain?

I’m watching youtube videos on terrains… do the designers place each individual tree and log down by hand, or is there some type of generator for it?

And does anyone know how to make this awesome grass? Rolling Hills - real-time grass rendering - YouTube

Trees
http://docs.unity3d.com/Documentation/Components/terrain-Trees.html

Grass
http://docs.unity3d.com/Documentation/Components/terrain-Grass.html