"Do people usually complain to the landlord?"

I'm a landlord: yes they do. And yes, I take it seriously.

"And if so, do the people making the noise find out who complained?"

I don't tell them who complained. I get the tenants to call me when the noise is going on and evaluate the conditions myself. Or I have them call the cops and complain about noise ordinance violations.

For one obnoxious group (the original tenant left some immature friends to "take care" of his place while he went on vacation) I showed up when the neighbors called, saw the wild party, and called the cops myself. One cop showed up, called for backup and they arrested a lot of underage drinkers and those who bought them the booze. It cost the oringinal tenant his lease and several hundreds in damages - the judge was not willing to let him blame the fritnds and leave me with the repairs: she told him to pay me and get his "friends" to repay him.

"How late is acceptable for noise level?"

Depends on the lease, and the town ordinances, but 10PM on a weekinight and midnight on a Friday/Saturday is about my limit.

"And what usually happens if you get a noise complaint?"

Worst case, repeat offenders ... swift eviction! I make it clear in the lease that renting is not a valid reason for keeping the neighborhood awake, and that repeat calls from neighbors will result in a quick trip to Justice court and them having to look for another apartment (with an eviction on their record).