As the name suggests, this is not an area populated by Chinese people but by Mongolians.
The history of this area is varied. The Chinese emperors ruled the area, then they had to give it up, there were new occupations by China, then control by the Mongols again and newly occupied by China till today.
Below you can see an excerpt from Wikipedia starting in 1945.
The InnerMongolian People’s Republic was founded shortly after the Second World War. It existed from 9 September 1945 until 6 November 1945.
People’s Republic of China
The Communist movement gradually gained momentum as part of the Third Communist International in Inner Mongolia during the Japanese period. By the end of WWII, the Inner Mongolian faction of the ComIntern had a functional militia and actively opposed the attempts at independence by De Wang’s Chinggisid princes on the grounds of fighting feudalism. Following the end of World War II, the Chinese Communists gained control of Manchuria as well as the Inner Mongolian Communists with decisive Soviet support and established the Inner Mongolia Autonomous Region in 1947. The Comintern army was absorbed into the People’s Liberation Army. Initially, the autonomous region included just the Hulunbuir region. Over the next decade, as the communists established the People’s Republic of China and consolidated control over mainland China, Inner Mongolia was expanded westwards to include five of the six original leagues (except Josutu League, which remains in Liaoning province), the northern part of the Chahar region, by then a league as well (southern Chahar remains in Hebei province), the Hetao region, and the Alashan and Ejine banners. Eventually, nearly all areas with sizeable Mongol populations were incorporated into the region, giving present-day Inner Mongolia its elongated shape. The leader of Inner Mongolia during that time, as both regional CPC secretary and head of regional government, was Ulanhu.
During the Cultural Revolution, the administration of Ulanhu was purged, and a wave of repressions was initiated against the Mongol population of the autonomous region.[29] Among the victims in Inner Mongolia, 75 per cent were Mongols, even though they only constituted 10 per cent of the population.[30] In 1969, much of Inner Mongolia was distributed among surrounding provinces, with Hulunbuir divided between Heilongjiang and Jilin, Jirim going to Jilin, Juu Uda to Liaoning, and the Alashan and Ejine region divided among Gansu and Ningxia. This decision was reversed in 1979.
