I'm not sure how the curvy shading was done, but the lines following edges looks like the results of vectorising the output of an edge detector: https://en.wikipedia.org/wiki/Edge_detection
Thanks, I'd figured it was something like that. It's very impressive how it works, he has more examples in his feed. Sometimes it looks more like there's a conversion to a sort of "visual direction" in the image which the lines follow, not just edges.
If you follow level curves as well as edges, you can get that visual direction effect. I think people may also do shading in directions similar to nearby edges.
