In digital cameras, when you push the button, generally a photo is taken. Then you push the button again and a new photo is taken. But on Holgas, because it is a manual camera, the wealth of possibilities expands.
To take a photo with a Holga 120 medium format camera, you press down the shutter release on the right-hand side (when holding it) of the camera lens. All that does is open and close the shutter. The action does nothing else. It does NOT advance the film.
That means you can point the camera at something and pull down the shutter release. Then you point the camera at something else and pull down the shutter release again. You now get two images super-imposed on top of each other.
In this example, I first took a photo of the warning sign for Purgatory Chasm in Sutton, Massachusetts. I did not advance the film. Then I took a photo of the entryway to the chasm itself. That overlaid the image of the chasm on top of the image I’d already taken of the sign.
That’s just an example with two overlaps. You can easily create fascinating images with multiple overlaps. Just keep in mind how the different areas of light and dark will impact each other.