Why is the Book of Changes called "Nine" in Yang and "Six" in Yin?

My personal understanding is that nine is the greatest in ancient metaphysics, for example, nine days is the highest day and the ninth five-year plan is the emperor. So he represents Yang and Tian. Six is for the emperor. For example, six, for example, six gifts. For example, the six gods in the family are all for the emperor. So he represents yin and earth. In fact, when studying, we should follow the ancient law and agree with Yang, nine yin and six. Learning the Book of Changes is encouraged. Follow-up: Daoyou, I heard that "among the gods, 67896 is the old yin and nine is the old yin, so six represents yin and nine represents yang". As for the ninth five-year plan, it is derived from the fifth five-year plan and cannot be used as a reason. On the contrary, the Book of Changes is the origin of its name, and then metaphysics comes from the Book of Changes! Yang, nine yin and six yin have existed since ancient times. I suggest watching some lectures first. After all, people have been studying the Book of Changes for decades, but don't be preconceived. I spit on the testimony of your grave: praise! Great! Very awesome! Never been so great!