Are Han Chinese descended from Mongols?

No. The Han nationality is the main ethnic group in China, a descendant of the ancient Huangdi and Yan Di tribes, and a descendant of the Chinese nation. "Han" originally refers to Tianhe and Yinhe. The Book of Songs says: "Where there is Han, there is light in prison." Han people are used to calling themselves Han people, because the Han Dynasty in China used to be called "Huaxia" or "Zhu Xia".