How Do People On Youtube Change Their Positions?

Whenever I'm on youtube I see that some people make videos to songs but they like first maybe as an example are on the right then are on the left because they take 2 different little videos of themselves like if the first few lines there just singing then there actually doing something that the song says by shooting 2 different little videos then putting them together in windows movie maker or whatever but how can they do that without having to walk towards the camera and stop it and then make another video for the other part of the song.How Do People On Youtube Change Their Positions?They don't stop the camera and then say something else in another position! They just edit it out....