How does gh get the texture of a rhino material
I don’t know how Grasshopper does it specifically, but here some scripts and tutorials related to render materials and render textures that might help you: https://jesterking.github.io/rhipy/ .
Can you have a look at this post? I discuss Rhino materials in Grasshopper towards the end of the post. Or are you asking specifically about how to get to the bitmap data used in a Rhino material?