If we do retain speaking language in China, comparing it to other cultures, English is still dominant in Australia, US, Canada, UK. Does this make them all British culture?
Yes, at least in that specific sense.
But China is not Britain. Britain is an island and not very big; as such the breadth of its cultural diversity is far less than that of China, which is the size of Europe. It is also clear that in Britain, the main culture is essentially English rather than Scottish or Welsh.
Which part of China, as mosodake brought up, is representative of "Chinese" culture? Mandarin is the main language now, but it has only existed in its current form for a relatively short period of time and was not well-propagated until about half a century ago. There are 7 categories Chinese spoken languages that are not mutually intelligible at all, like Cantonese and Shanghainese. In 1900, Mandarin-speakers made up a plurality but by no means an overwhelming majority. And they often spoke their own variations of Mandarin that might not be immediately intelligible if you weren't from that area. Basically, China and therefore the idea of "Chineseness" is incredibly diverse.
Some people argue that being Chinese stems from being ruled by the Chinese emperor, or by a polity that claims the same prestige. But what if the country finds itself in a period of chaos? Are the Chinese people no longer Chinese? This is why I say that the real definition of "Chinese" lies in the usage of Chinese characters. This is the only definte unifying factor. "Chinese" doesn't mean chopsticks, it doesn't mean dumplings, it doesn't mean pagodas, it means Chinese script.
Now of course, there need to be some considerations: Koreans, Japanese, and Vietnamese all used Chinese characters 100 years ago; Japan still uses them and the Koreans have only recently phased them out of common use. So were these people Chinese? I'm not going to say that they were, but the fact is that by using Chinese characters they connected themselves in a linguistic and thus a rather deep cultural sense to that multiethnic, multiregional country in East Asia we call "China". It's also worth noting that none of the three countries I mentioned fully adapted Chinese script to their languages; Japanese for instance is composed of many completely native words that aren't rendered in Chinese characters, as well as Sino-Japanese terms (and western loanwords). So they are not Chinese.
If you have populations in other countries who came from China, and write Chinese, then they are culturally Chinese. It's similar to how one can be both American and of another nationality, since the idea of "American" does not limit itself in the way that European concepts of national identity tend to do.
If I understood this correctly, If directly rooted/originated there, wouldn't anyone with ethnic blood Chinese qualify as well regardless if it was their parents/grandparents or great grandparents or more that were Chinese? Or did you mean only parents or themselves who came directly from China?
No. As I explained above, the idea of "being Chinese" is ultimately only concerned with the Chinese script and its derivative languages. In OTL the areas ruled by a central Chinese polity tend to correspond with the areas that use Chinese characters, but this is only incidental. If you do not use Chinese characters, or would not learn them if you were to become literate (since many Chinese, have historically been unable to read) in the area that you live in, you are not, generally speaking, culturally Chinese.