>>43237965I'm the guy who did the pretty mediocre ones like Salandit and Turtwig. I made them probably over a year ago but the method was something a long the lines of getting the Sugi artwork first, turning the opacity down, and then layering coloured objects on top where they would fit.
Google has this tool when searching for Images that you can click called "Colours" and you can pick any colour and it will only show results of that colour. Combine that with a search like "objects png" and you're good to start finding things. You'll get umbrellas or phones, even dinosaurs.
When I saw the art I could imagine some pieces fitting in there from the get go, like I knew beforehand I wanted a telephone acting as the mouth and cheek for Turtwig and a sticky hand for the markings on Salandit's back.
Most everything else is just seeing it in google images after the fact and realising it's a good fit.