All the youtube videos and blend files for the book are available free from the book site get-into-bender.com and you can read a sample pdf from there (it has about 5% of the pages hidden, but everything for these videos can be read). Of course if you want to buy the book it's only $7.50 and a proportion of the sale price goes to the Blender Foundation.
The first video is explained from page 276 to 282 and covers a similar process to Samuel. The second video on page 283 to 285 goes through painting textures to control the elevation of the terrain and the density of boulders.
As well as the blend file worked on in the video ch1401_geometry_nodes, there is a scene from chapter 2 of the book but with the geometry nodes boulder density controlled by vertex groups.