Should a physics material be added to the collider or the rigidbody?

Hey, I think I found the answer to your first question. If you assign a physics material to the rigidbody, then all attached colliders of your object will have this physics material. If your object has multiple colliders and you want to assign different physics materials to each of them, then you would assign the physics material directly to the colliders.

You can add a material to each collider and it doesn't have to be the same. Unlike the 3d physics system the 2d system doesn't allow to specify how the materials are combined. In 2d physics it seems to be always "multiply". Though in 3d physics the material can actually choose how to combine the values for each object.