Composition in Landscape painting

Currently reading this book Landscape Painting (Basic Genre)

Most famous landscapes have one element in common, there is foreground as well as background interest. Foreground is typically detailed and the main focus. Background is is usually depicted by small mountains or distant houses. But few famous painters have their unique ideas. Look at these compositions below.
'Old Battersea Bridge' by Whistler

'Tahitian Landscape with mountains' by Paul Gaugin

'La Montagne Sainte-Victiore' by Cezanne

'Nichols canyon' by David Hockney