Parental Guidance: to what extent is it practiced?

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I'm not a parent yet. But I have seen how my nieces and nephews watch tv. You can't really stop them from watching TV especially when they are at home. However, not all shows are good for them, that's why some shows are rated General patronage and some parental guidance. Children just pick anything from the shows, such as language and actions and which influence their behavior, too. How do parents, or guardians, really practice parental guidance? are we always available to explain to them something they watch that's not good? or do we really filter what they watch? like, we only choose the shows they are allowed to watch? how to do you do it?