They don't detect _copyright_, as far as I know. Videos are sometimes blocked or muted because a copyright-holder claims a violation, but this is a mass action based on the video or song itself, NOT on the copyright of the song. Because of this, you occasionally see videos that are posted by a legitimate user (for example, the band itself) that are deleted as part of a sweep, then restored later once it becomes clear that it's not supposed to have been deleted.