In those tutorials it’s covering using landscape material. The last one is using a mask material to allow you to see through those parts of the mesh painted with the mask material.
If you want to cut a hole in the Landscape you’ll need to do the following:
In Landscape Mode > Select your landscape > Change the mode from sculpt to “Management” > select “Delete” mode
With this mode enabled you would use it the same way you would sculpting.
This mode will select the component squares that make up your landscape and hold CTRL + LMB to delete the square component.
You can use other meshes/landscapes to be under this hold to create your cave-like area.
If you have any other questions feel free to ask!