The Mythical Problem of Yi Xiaochuan and the Thousand-Year-Old Demon

When Yi Xiaochuan appeared in the modern world, was the thousand-year-old demon also in the modern world? Is that thousand-year-old demon the person Yi Xiaochuan returned to the Qin Dynasty and lived to the modern world? Is there only one Yi Xiaochuan in the world when Xiaochuan returns to the Qin Dynasty? Didn't Ogawa and the others only look for Ogawa for a short time? How could it have been more than 2,000 years?