Assuming they're on different dies, it would be the same way they connect the thousand or so pins (of an FPGA or CPU) from the die to the BGA/PGA/LGA package: i

Assuming they're on different dies, it would be the same way they connect the thousand or so pins (of an FPGA or CPU) from the die to the BGA/PGA/LGA package: interposers with wires whose widths are in the nanometer range