Already exists, same as is used for any other soundproofing. Rockwool insulation, resilient channels and a second layer of drywall, mass loaded vinyl, acoustic

Already exists, same as is used for any other soundproofing. Rockwool insulation, resilient channels and a second layer of drywall, mass loaded vinyl, acoustic panels and tapestries (can hide some more mass loaded vinyl in there too), acoustic adhesive, scored screws to kill floor squeaks. They're all quite expensive but hey! Very DIY accessible. For exterior noise the biggest bang for the buck is replacing windows. I had some soundproof windows put onto my previous house and you could close the door on a parade going by and not even know it was happening.