Most of all it's just that there's written rules, and that you can point at them if you feel things are too loud. With immission a lot is already considered when building new houses, train rails, streets, for example.

With cars and motorcycles there's also emission guidelines, and you have to register aftermarket changes with the inspection authority. And if the police catch you with unregistered alterations they can fine you, ask to show proof of registration within a week, or even impound your vehicle on the spot.

And if there's a e.g. noisy party in your area, you can call the police (a rite of passage for every German – your first noise complaint ("Ruhestörung")) and they will ask nicely one time, if they have to come a second or third time they might confiscate your stereo, disband the party, etc. But it's just their decision what's too loud, they won't take measurements.

If it's something that's not immediately obvious (e.g. dogs constantly barking), or the government is... less eager to act (e.g. airports) then it can evolve into a tedious and nerve wrecking legal battle. Which frequently happens.