Here I simply give an an example of what can be done and leave the rest to your imagination.
First of all, let us look at three of my recent holiday photos; none of them are very startling but are simply snaps taken while the camera was in my hands.
A ferry leaving harbour
A view of distant volcanoes (dormant)
A fort on a hillside (originally for protecting the populace from pirates)
Now to proceed ... I was going to use the centre image as my background so it can stay as it is - for now. The ferry I wanted without any sky, so I selected the sky area and deleted it creating a transparent space above the sea/ferry line.
I also wanted to use the fort as my foreground and so did a similar thing to that photo.
I now have the wherewithal to start to build my imaginary landscape. I open the background in photoshop, and paste the other two photos as layers. The order of these layers will be as already suggested. The volcanoes are the background (and I later copied this layer to be able to adjust it), the ferry is central, ie visible above the background. And the fort is on top.
And there we have a new, imaginary landscape. A boat floating past on a non-existent lake in the middle of a desert landscape in Lanzarote. But does it look right? The boat is very large compared to the fort, especially as it is in the distance. Also the foreground might be expected to have more contrasts in the bright sunlight.
In the last image, I have taken back the contrast in the background and bought the foreground up by darkening and adding contrast.
I have flipped the ferry horizontally and reduced its size. This gave me a small problem in that it did not fit across the edges, so I had to cut and paste a strip of the sea in place, however my last edit actually removed the need for this. I enlarged the fort to make the perspective a little more realistic. Could do better you might say, but it is thte first example I have tried and took the first couple of images I came across. But I hope you can see the technique.
I will be experimenting with this and may come back with more examples and examine the possibillities in more depth.