Interposer
The slab of silicon under the chips whose only job is running wires — denser than a circuit board can manage and cheaper than another logic layer.
An interposer is a passive substrate, usually silicon, on top of which several separate dies are mounted. It contains no transistors, only conductive traces and through-vias, but because it is manufactured with semiconductor processes those traces can sit orders of magnitude closer together than on an ordinary circuit board. The arrangement is called 2.5D packaging: the chips lie side by side but connect as though they shared one piece of silicon.
This is what makes HBM practical. A memory stack with thousands of connections cannot be wired across a circuit board, but it can be wired across a few millimetres of interposer. TSMC's CoWoS is the dominant variant, and it has become a bottleneck in its own right — the supply of packaging capacity, not of silicon, has at times limited deliveries of AI accelerators.