People get 120db by having enough subs and enough power that they can turn it up "louder" than reference.
A more technical explanation.
Bits are encoded on a disk, processed by the DSP, and sent to a DAC/preamp. At max, that signal is at -0db, which might be 1-2v. That signal gets sent to the sub amp with a 36-48x voltage gain. Fort those folks getting >115db output, they have another pre/pro between the preamp and power amp that manipulate and boost the signal, maybe even to 3-4v. Now with that same fixed gain structure of the power amps, they're able to send enough power to the subs to surpass 115db.