'To yung laging sinasabi niya (anong laging sinasabi? Ito.) Gan'to n'ya niluto yung ulam (anong niluto ng paganito? Yung ulam) If its to replace a noun or reaffirm/remention a noun that was mentioned earlier or will be said later, its only "yun". As another commenter said its short for something different to yung. More @Wikipedia
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