Texture in the shaded areas was done in post.
You can use textures and materials if you want. In this image even the wood grain textures are hand drawn.
I used to use watermark images quite frequently but generally now prefer to add backgrounds and that sort of thing in post.
It’s also worth keeping in mind that image export size has an impact on the appearance of the style. The image below shows the difference between various export sizes using the same style.
If you want thinner lines, export large and resize the image in your image editor.