This is from the last section of my book get-into-blender com you can read the pages referred to below in the preview sample available on the book website.

The first video is explained from page 276 to 281 and covers adding and rotating objects over a surface. The second video on page 282 to 284 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.