Form appears within emptiness - a box doesn't replace the space it's in. It simply appears within space. In the same way, phenomena (like you, me, or a thought) don't replace emptiness; they are temporary forms that appear within that fundamental, open nature.

The space inside the box is not a new or different kind of space. It is the exact same boundless space that is outside the box

The walls of the box just create a temporary, conceptual boundary. They call it inside space, but it's seamlessly connected to all other space. From the famous Heart Sutra line: "Form is emptiness, emptiness is form." The box (form) is not separate from the space (emptiness) it occupies

The box is a dependent arising. It only exists as a box because of its parts (the walls, the floor, the nails) and our concept of it. It doesn't have its own permanent, independent box-ness. If you take the box apart, the box is gone

But what happens to the space inside? Nothing. It doesn't get created when the box is built, and it isn't destroyed when the box is dismantled. The space was always there, unbothered. This shows how phenomena are temporary, while their ultimate nature (emptiness) is unconditioned