The content owner is different from your YouTube user account. You need to be signed in as a content manager to act on behalf of the content owner. For Content ID, the content owner ownsthe assets in YouTube’s rights management system and is the entity that claims videos and sets their policy.
YouTube videos are viewed by people which are powered by YouTube, they scan videos for Bad language and so on.
If they find any bad language they won’t let the creator of the video monetize it and will mark the video PG so that only people with google accounts can see the video to prevent any underage people watching the video. If there isn’t any bad language and the video gets reported by viewers the video will be checked manually and will be also marked age restricted.