It was me, actually! I used Procreate on the iPad with True Grit Texture Supply brushes, they do excellent textures. I overlaid some stock photos and the “path” down the middle of the Leviathan is a combination of two public domain artwork. It was put together in Photoshop in the end, that’s where I was playing around with the blend modes.
Layering the effects and getting the gradient map colours right took most of the time apart from “sculpting” the serpentine body of the Leviathan so that the mass and weight is consistent. The linocut effect was a happy accident from the sculpting bit, I leaned into it to have more of a handmade effect to it.
Hope that helps.