Physical layers
So images like this one consist of layers, lots and lots of layers. Not digital layers in photoshop though, layers of thin plastic sheets with hand altered images on each one. Then in this example placed over b&w photographic paper then weighed down with a glass sheet and exposed with a UVB bulb for several hours.
Cliche Verre/Sun Print - Ilford Fiber Glossy Paper exposed to a plate consisting of Carbon, alcohol, ink, engraving and film