Absolutely, and in fact there still are.

Officially, there are 56 ethnic groups in China. One could argue there are more or less ethnic minorities (ethnic groups are quite difficult to define, after all), but there are a lot. However, 91.5% of people in China are (defined as) Han. When you think of "a Chinese person" you are almost certainly thinking of a Han. Some minority groups look almost identical but are defined seperately for historical/religious reasons (for example, the Hui), some are similar looking but different enough that you might be able to tell (for example, Koreans), and some look very different (for example, the Uighers).

But, even with the Han, it's not quite so simple. Take a Han from Heilongjiang (in the far north) and one from Guangzhou (in the far south). The northerner is probably noticeably taller, paler and speaks Mandarin. The southerner is probably noticeably shorter, darker, and speaks Cantonese (though these days can probably speak decent Mandarin, as they have to learn it in school). Northern traditional folk religion focuses more on venerating various gods. Southern traditional folk religion focuses more on ancestor-worship. Their cuisine is quite different (actually, there are generally recognised to be 6 different kinds of Chinese food). So why are they defined as one ethnic group? Now, actually, they're not massively different. Both are more similar to each other than they are to a Japanese person or a Mongolian. But ethnicity isn't as cut and dried as government censuses say it is. (This is not to mention that there are plenty of mixed-ethnicity people who probably tell the government they are Han and tell their friends they are Manchu or Mongolian or something, and then there are some families, especially Manchus, who just told the government they were Han after the overthrow of the Qing Emperor because they were worried about anti-minority prejudice).

But even saying all this... yes, they are all Han. Han are one of the biggest ethnic groups in the world. And while they may vary from place to place, the differences are, as I said, not necessarily huge. The Han, essentially, absorbed many smaller ethnic groups, or conquered them and culturally assimilated them (sometimes coercively, sometimes peacably).

"China", in the way that the Han traditionally think of their civilisation, started in what is now it's central eastern region, under the Zhou dynasty, in the 1st millenium BC (this is a simplification, but we'll go with it). Then under the Qin and Han empires (221BC-220AD), it spread down into what are the Cantonese-speaking areas of south China. So these regions were, at that time, quite different from northern China, but since this was about 2000 years ago, they've slowly been assimilated (though there are still differences between north and south).

In fact, you'll notice that one of those dynasties was called the Han; the ethnic group is named after them. Essentially, think of the Han dynasty as essentially creating the Han ethnicity, by a)spreading a certain culture, and b)providing a large empire where people could travel and intermarry.

During the following 3 kingdoms period, what is now south-east China (Yunnan, etc) came under the loose control of Chinese kings, but this wasn't retained by the Sui dynasty. At this point, there was still a definite difference between the ethnic groups, as you can see mentioned in classics from/about the period.

So, for most of the 1st millenium AD, "China" was what is now east-central and south-east China. For 1000 years, all these people lived in "Han" societies*, and merged into the "Han" ethnic group. One notable point: under the Tang dynasty, what is now Xinjiang came under the loose control of the Chinese Emperors, but they didn't exert strict control, and Han did not move there. So the locals did not absorb Han culture.

(*The Northerners were frequently conquered by various "barbarian" groups like the Jin and Liao, but they tended to adopt Chinese customs after a while)

Then, in the 13th century, China was invaded by Mongols, who conquered it (the north first, then the south later). The Mongols also conquered Tibet and extended China's north-west borders. And of course, now Mongolia and China were under the same Emperors.

The Mongols adopted Chinese culture a little, but not significantly. However, they didn't rule for a very long period, and were replaced by the Ming, a Han dynasty. The Ming didn't retain all of the Yuan Empire, but they did retain most of it, and spread Han culture north and north-west.

(One notable point is that the Ming had a very complicated Patron-Priest relationship with the Lamas of Tibet. That's a whole 'nother post, but the important thing for this is that they did not significantly interfere with the Tibetans way of life, and did not really assimilate them into Han culture.) China was then conquered by northern "barbarians" again, this time by the Manchu Qing Dynasty. The Manchu, like the Yuan before them, did not promote Han culture, but they did expand China's territory, allowing Han to move into some new areas (though Han were banned from Manchuria). Some of this was later lost when the Qing fell (like Inner Mongolia).

So, this leads up to 20th century China, and its current borders. You have the eastern areas, which once had all kinds of ethnic groups and languages, but which were all ruled by "Han" rulers for about 1000 years, and so generally merged into one "Han" ethnicity. Then you have the areas that came under Chinese control in the 2nd millenium, many of which, as I have mentioned, were not assimilated for various complex reasons. For this reason, the western regions, as well as the far north (Heilongjiang) and south-west (Yunnan), have a lot of ethnic minorities.

If you're confused by all the dynasties, have a look at this map: https://en.wikipedia.org/wiki/History_of_China#/media/File:Territories_of_Dynasties_in_China.gif I should warn you, this is a wikipedia map, and it is very simplified. The relations between Xinjiang and Tibet (as well as Korea, Mongolia, and northern Vietnam), and the various Chinese dynasties are actually very complicated, as central Chinese Emperors exercised tributary dominion over various regions in a way that doesn't neatly fit into a "In China or not in China" Westphalian system of borders. But it's useful if you want a basic idea of what's going on.