Here are the biggest noticeable differences between OTL China and TTL China.
1. Due, to be blunt, to a more active policy of genocide against nomadic barbarians, Genghis Khan is butterflied away.
China still has northern conquest dynasties several times, but it never gets fully conquered by them.
2. Chinese literary culture is less well-developed, and stories like Journey to the West have a different moral slant to them.
3. The Hui Muslim population doesn't exist because the Muslims who converted them IOTL weren't allowed in.
Buddhists still exist, but they're only tolerated in designated vassal states like Tibet, Dali, Korea, Japan, etc.
Daoism is considered morally degenerate for rejecting the strictures of duty imposed by the State, even if it has influenced Legalism quite a bit.
Christianity and Islam are forbidden because they deny the Emperor's divine status and place the highest good outside the state.
4. China is very xenophobic and over-the-top racial supremacist.
This doesn't prevent the state from hypocritically using foreign technological advancements and passing them off as their own inventions.
5. After seeing the tribute their vassals bring them, the Chinese send out fleets to Africa and Australia to find those exotic animals and resources (giraffes, ivory, precious woods, kangaroos, spices, etc).
Small outposts are established, but not major colonies. Nevertheless, all of the ocean east of Sri Lanka is a Chinese pond.
7. China is a very totalitarian society.
Public intellectualism is discouraged, foreign religions are not tolerated in Chinese territory, and loyalty to the State and Emperor is required to be a moral person.
There is remarkably little ideological diversity, and traditional Chinese religions have largely been coopted by and woven into the Legalist moral framework, so you can't escape its grip even in your prayers.
Most people are either farmers or craftsmen, and it is considered virtuous to toil in the fields or in the army for the good of the State; artisans and people who create luxuries are considered necessary evils, if only so that the State can create an artificial distinction between itself and the people it rules over.
Private citizens have no rights that can't be abrogated by the state, and the state incurs no moral penalty for doing so.
Private citizens have to obey extensive sumptuary and dietary laws which distinguish them from their social superiors and the State; even nobles have an upper limit on how opulently they can live.
Social advancement is controlled by the state so it can only be done through the army and bureaucracy, and merchants are shunned because unlike every other class they create nothing and become wealthy without serving the state.
Science is only promoted in the fields of astronomy, agriculture, warfare and architecture: everything else doesn't exist, and the scientific method is discouraged because it is antithetical to the idea of obedience and ritual-induced habits.
8. Legalism has turned into a religion of sorts, where the Emperor is a semi-divine being sent by Heaven to rule China, China is superior to all other lands and the Chinese people superior to all other peoples, and the good of the State/Realm is the highest moral good above all others.
9. Politically, Legalism is still all about operating the Two Handles as Han Fei put it IRL: create rewards and punishments, spell out the rules for everyone to know, and by following these rules people will become accustomed to doing the expected behavior without thinking about it and therefore without any need for state intervention.
China IOTL exhibited a lot of the features of a modern state during its medieval era, so this is pretty much a medieval Chinese version of 1984.