What region do you referring to? Can you give me specific regions which region Han Chinese settled for millenniums?
I don't know al of them (many were probably villages that either no longer exist or merged into other units), but the area around Chifeng had Han and pe-Han (that is the peoples who became the Han and related Northern Chiense) settlements, in fact the ruins of the 'oldest Chinese village' are found there.
Aside from that the Han have lived in different areas intermitently for a very long time; over 60,000 Han were settled in what's now the Ordos region in the 3rd century BCE while in the 2nd century BCE separate settlements were established in what is now Bayannur while their were other settlement periods (though AFAIK not as large or extensive) in the same broad region in the 1st and 2nd century CE.
Now, not all of these settlements lasted, however some did, indeed during the Sixteen Kingdoms period some of the Kingdoms in the region were ruled either by Han dynasites or by dynasties with some Han descent.
Chinese and Mongolians we enemies, so they never tolerated each others.
What I understand and based on limited sources, there wasn't Chinese migration to Mongolian lands due to hostile geography and climate.
It's really depended on the time period, however the Mongols and Han generally did'nt go on genocidal campaigns against each other when they gained new lands.
As to geography in climate, again it depends, some parts of Inner Mongolia are conductive to settlement while others are'nt.
Also settling inside Mongolian borders were very dangerous as they would raid and kill Chinese (guess why Chinese built Great Wall, enormous structure).
Actually it was the policy of many Chinese dynasties to settle the border regions with Han to protect the Empire as a while by ensuring loyalty and potential manpower.
Also, the Great Wall was not a single thing, rather it's the result of various different walls built at different times over centuries, usually against different groups.
Where there is sources and facts that point there was hell of Han Chinese who overwhelmed Mongolians? Or do you think since 2013, 75% of Inner Mongolia is Han Chinese, so was in 1850 and earlier?
Oh don't get me wrong, I'm not saying Inner Mongolia was large majority Han constantly, but rather that the Han have had a presence there for a very long time and, due to immigration/settlement starting in the 18th century, became the majority in the 19th century and had become a large majority by the late 19th century.
As an aside, the number of Han has actually gradually decreased; while in 2000 the Han comprised 79.2% of the population, back in 1947 they comprised 83.6% of the population; as I've been unable to find comprehensive English language results for the 2010 census I can't say to what degree it's continued.
Before 1750's Inner Mongolia was what is OTL Inner Mongolia minus North Eastern regions (which was under General of Heilongjiang).
Before 1650, Inner Mongolia included OTL ROC province of Rehe and Liabei as well as Nunbei and little more lands.
In the former case then the Han, while still a minority, would've been larger than they would've been with said territory (which was predominantly Mongol and Manchu) while in the latter case it would've had a much larger Han population.
First I thought you have some good sources about Mongolian population during 18th-19th century. I was even glad i finally can find good source, since finding Mongolian population and Inner Mongolian Demographic data is very hard to find.
Unfortunately, as it was'nt really something the Chinese or anyone else in the area cared about at the time their's not really alot in the form of ethnic census data, however I do have a few links that provide base population numbers, and possibly some others that provide more that I've never looked at.