/u/Frigorifico has written a lengthy response here, but I think their perspective is missing several key points and is at times flat out wrong. They are essentially repeating a claim which I have seen several times, which is that the creation of "Han identity" is a sort top-down government initiative to classify people as Han, in order to create an appearance of unity.

In fact, despite the at-times dramatic regional differences, Han people self-identify as a single ethnic group, and have done so for hundreds of years. Han nationalism in its modern form is of course a recent phenomenon, but even as far back as the Yuan dynasty there was a very clear sense among literate classes that they all belonged to a single "Han identity", as opposed to outsiders like the Mongols.

The key issue here is that "Han Chinese" define their ethnic identity in a way which is rather different from nearly any other ethnicity. In turn, this makes it possible for Han Chinese to view themselves as a single ethnicity despite having substantial internal diversity that other groups of people might see as ethnic differences. However, given that ethnicity is first and foremost a social construct, I don't think any outsider has the right to say that this widespread self-identification is not valid.

Part of the issue is that the Chinese languages as a group are fairly unique in that they are the only widely used languages to have a symbolic written system. The standard definition for a language is that two tongues are dialects if they are mutually intelligible, and languages if they are not. However, this definition fails for the Chinese languages - because the language is symbolic and not phonetic, the Chinese languages are mutually intelligible when written, but not when spoken.

If we go by the spoken language definition of language, this actually only makes the linguistic situation in China more confusing rather than to help us to better understand the Han people. The spoken languages of southern China are one of the classic examples of what is called a dialect continuum, in which languages of adjacent regions are mutually intelligible but as you cover larger distances, the differences accumulate and languages gradually become unintelligible. As a result, there is really no sensible way to divide these into linguistically distinct "languages" in which each group consists of mutually intelligible dialects and each language is mutually unintelligible with each other. Chinese scholars have a traditional classification, but partly because of the fact that it is not a clean classification, people do not really strongly identify with the language/dialect they have been classified into. Moreover, similar phenomena exist with other cultural differences like cuisine.

As a result, even though on paper Wu Chinese are a minority of 80M people, no person of Wu Chinese descent (myself included) will ever give you a straight answer on what it actually means to be "Wu". A Wu person would instead identify by their family hometown (in my case, Ningbo), with a clear expectation that people from nearby towns will be more similar and easier to communicate with, and people who are from further away will be more different. Unless we want to classify each town as its own ethnicity, the most sensible grouping is to simply group all "Han" people together - and in fact this is exactly how Chinese people have traditionally viewed themselves.

As a result, when /u/Frigorifico say sthat "60% of the population does speak Han", they are flat out wrong. Han (汉字 in Chinese, literally Han characters) is by definition an exclusively written language, not a spoken language. This is equating "Han" with Mandarin, but the two are not the same thing. In fact, 92% of people in China identify as Han and are native speakers of one of the Chinese languages which uses 汉字 as its writing system, even though not all of them are native Mandarin speakers (the vast majority of non-native speakers still speak Mandarin at a pretty high level as a second language).

This is especially important because Han people have traditionally defined their culture and civilization in terms of a shared literary, philosophical, and governing tradition. As a result, there is a clear belief that all people who can read and write Chinese can partake in this common ethnic identity even though they may speak unintelligible languages or eat different traditional cuisines.

Returning to the heart of the point, "ethnicity" is fundamentally a sense of shared identity. At its core, China's long history as a unified country has created a strong belief that the Han Chinese people are a single people, compared to India which has been unified only recently and was constructed by foreign conquerors as opposed to through a shared unifying push.