Hello!
I have a mesh and I want to be able to detect were a raycast hit it and then extrue or flatten that point how would I do this?
Please help thank you! 
A ray hit will return a world space point. You can transform it to the mesh’s local space, (see TransformPoint/InverseTransformPoint in the Transform script API). The next step is to map that point to the Mesh, and perform whichever operations you choose.