• "The imperial overnment (of the Xiong Nu) was at its most powerful and highly centralized when it conducted foreign affairs and organized military campaigns. It was least powerful and most decentralized at the local level where it ceded political power to the indigenous tribal leaders who were responsible for handling most domestic problems."

• "The imperial level of government was financed by drawing on resources from outside the steppe, not by taxing the nomadic animal breeders from within the empire."

• "Nomadic conquerors who created empires along the lines of the Xiong nu and used them to exploit China produced powerful states that survived their founders. Those nomadic conquerors who failed to see that a nomadic state required the support of resources from outside the steppe had their empires collapsed or suffer a power vacuum at their deaths."
  

• The notable successes founded along Xiong nu lines were those of the Turks and Uighurs who dominated the steppe from the middle of the sixth century... Both successfully forced China to fund a complex state by extorting subsidies and controlling a vast quantity of silk.
  
  In 840, the Kirghiz successfully destroyed the Uighur state and occupied its heartland, but they proved incapable of recreating the complex relationship that the Uighurs had established with Tang China, which funded the Uighur state. AS a result, no Kirghiz state ever came into existence...nor attain any power over the rest of the steppes.
  

As is pointed out, China was th emost important of these settled peoples to the nomad states.  The tribute system served as an easy way to extract resources without costly and possibly societally destabalizing occupations.  Nonetheless we should not forget the other civilizations that also had an apreciable impact who also had a steppe border.

Ive also heard about western steppes being more fertile.It doenst explain why the eastern steppe armies seemed to have higher populations than the western ones. 

Shinai

"Despite their nomadic backgrounds, without them China as we know it today would never exist."
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This kind of statement is base on nothing but assumption. How do you know that is the case? And how do you prove it?

"Mandarin or Putonghua was developed later in the Qing dynasty, this language is till today the official language of the PRC."
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'Mandarin', for whatever reason the westerners nowadays call this the official language of china as 'Mandarin', indeed, it has not much to do with the Manchu people. "Mandarin" is a local dialect of beijingers, where the majority of Beijing people of Ming period were coming from the south, the surrounding area of Beijing and mingle with the local people.

"The Mongols and Manchu’s united China”
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The Mongols conquer China eventually, is not for the purpose of united China, but for their land and material ambition, for a selfish purpose only, same as for the Manchu people take over China.

'Defined the borders of present day PRC, which is a subset of their
Respective empires'
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Foe the border issue, Mongols or Manchus, take the land is their land, also for their own beneficiaries only. Border is their border, their 'China's border, since China is their China.

In history, every dynasty changes in China, new border of China, size of the land the the new dynasty is able to control or rule, it depends on the military power of the new dynasty, and the ambition of the new dynasty.

For the contemporary history, actually, this kind land, military power, border, and ambition relationship, it apples to evry country and the whole world.

Manchu-Ching once ruled outer Mongolia, and a large area of northern Heilongjiang. When in the turns of last century, Manchu-Ching collapsed, Chinese people were so week at that time. New the china was in civil war, no regime or power was able to fight against the strong Russia, so Russia was manage to make outer Mongolia independent, and took a very big chunk of northern lands of Heilongjiang.

So, new China's new borer, is define by new China's own strength, in relatively to the out side power. But it is not define or set by Mongols dynasty or Manchu dynasty.

China's border has a lot to do with Qing dynasty, as for mongol or manchu uniting china, it may not have been their purpose initially but that was the result.