Why don't colliders work?

I know I'm not the first to ask this question, but I haven't found any answers that address the issue.

I'm using Unity2019.2.0f1. All objects with colliders have rigidbodies. All colliders are box colliders. All rigidbodies are set to detect collisions continuously. UseGravity is true for all rigidbodies. The positions and rotations of the wall, ceiling, and floor rigidbodies are constrained.

Nevertheless, on Play all objects just fall through the floor. What am I doing wrong?

 

The world geometry should ideally use static colliders, i.e. have no rigidbodies.

 

Don't objects need Rigidbodies to be effected by Physics?

 

Dynamic objects should. Static objects should just have colliders. Here is the relevant documentation:
https://docs.unity3d.com/Manual/CollidersOverview.html