How to get the collided plane of a mesh, not just the object,How to get the collided plane of a mesh

Note that OnCollisionEnter receives a Collision as the parameter, and the Collision has the .contacts property, which contains an array of ContactPoint. Each ContactPoint has more information about the collision that you can use, such as the .normal, which is the collided plane orientation of the hit Collider, which you can use to reflect your objects.

You might need to do some filtering to retrieve the ContactPoint you're interested in, though, but the example code here (https://docs.unity3d.com/ScriptReference/ContactPoint-normal.html) should point you in the right direction.