Question about corner piece constraints in cube model

[sszxc]

Thanks for this amazing cube model! I have a question about how the corner pieces are constrained.

I notice that corner pieces (like pX_pY) inherit from the cubelet class which provides a ball joint:

<default class="cubelet">
    <joint type="ball" armature="0.0001" damping="0.0005" frictionloss="0.001"/>
    <geom type="mesh" condim="1" mesh="cubelet" euler="0 0 90"/>
</default>

I think corner pieces are child bodies of the core. However, I'm still unclear about how are corner pieces physically constrained to maintain their position relative to adjacent pieces? What prevents corner pieces from falling off? Is the constraint achieved through the mesh geometry, joint properties, or some other mechanism?

Thanks in advance.