There are certain sounds that may not be captured on the filming site, such sounds like a punch, or the rustling of clothes. These and many other sounds are therefore reproduced in the studio by a foley artist.
You watch a character or two singing so well in the movie. You know in reality they might sound okay, but not as great as they sound in the movie. Yes, it’s no magic. They often use a technique called ADR [Automatic/Automated Dialogue Replacement]. And there is nothing automatic about this process, you have to put in the work.