Chips are made from silicon wafers that are ~0.8mm thick. The transistor layer is built on one side of the wafer (because the transistors need to be built from epitaxial silicon, that is, where all the atoms are neatly arranged, and you cannot deposit a perfect epitaxial layer). Then the metal layers are built on top, by depositing copper and dielectric (traditionally silicon dioxide, that is, glass, but these days some more exotic stuff is used too).

Then that chip is flipped around, so the transistors and metal layers face the pcb. Most of the chip on the upside is just bulk silicon, and there for structural and heat spreading purposes, it's not needed for operation.

When chips are built out of multiple stacked layers, typically they thin the chips that go below and leave a thick bulk layer on the topmost one. IIRC, for some AMD chips they needed to use a separate, unprocessed bulk heat spreader layer for bonding reasons.