I tried the code on the Unity Scripting Reference to see how CalculateFrustumPlanes() works, yet it gave me these really weird planes that were not representative of the view frustum at all!
What’s going on here?
They do work, but you interpreted the result wrong. The returned planes are infinite mathematical planes, not polygons. The example in the docs is actually misleading. Drawing a quad mesh for each plane makes no sense this way. There have been a lot questions about that topic. See this question for how to create a mesh that represents the camera frustum.