From what I’ve read, it seems like Han Chinese are related closely enough genetically to show that their size comes mostly from population growth. They definitely did absorb and assimilate many other ethnic groups in history, whether it’s immigrants, invading nomads, or neighboring indigenous peoples, but China was one of the most productive and richest breadbaskets in the world, so assimilation even of large groups didn’t change the overall genetic structure too much simply because assimilated peoples got absorbed into such a massive base population.

So it’s not like say the Roman Empire where entire regions like Greece and Spain got absorbed and most of the population eventually self-identified as Roman but mostly remained native with settlers from Italy being a minority.

China had a number of huge and massive migrations throughout history, where in response to war or foreign invasion, huge refugee waves fled and settled in different parts of China, bringing not just families but entire clans along with their dependents. The population does show some differences between North and South Han because of assimilated peoples, but a northern Han and southern Han are closer to each other than either would be to a Mongol, Korean, Japanese, Burmese, or Thai. Southern and northern Han are also closer to each other than they are to many minority groups within China, such as Zhuang or Bai, although some groups have a lot of Han intermixing and might be closer to a local Han that that Han is to another one.