Something that I myself am guilty of but I suspect that at least in the European context, Asian (and sometimes African) cultures are often treated as interchangeable. There are some similarities but also profound differences. Consider the cultural differences between China, Mongolia, Japan and South Korea. As far as I know about them in the Sinosphere, the latter three were Chinese colonies (Japan even had Chinese aristocratic families at some point). At this point, China’s got Inner Mongolia (separate from Mongolia proper).
Both the Japanese and Chinese use Chinese characters, Mongolia uses Cyrillic (same with Turkmenistan and both were part of the Soviet Union) and Korea uses Hanggul (though they did use Chinese characters before as did Vietnam which switched to Roman characters*). China’s been influenced by its Central Asian neighbours, not just Mongolia (which a part of it exists in China) but also by Manchurians which came to dominate China right down to influencing its fashion sense.
The ethnic composition of China overlaps a lot with Mongolia and Russia. Especially when it comes to ethnic Chinese in Mongolia and ethnic Mongolians in China (inner Mongolia) as well as Buryats and Kazakhs being found in all three in addition to their own country (Kazakhstan which also belonged to the Soviet Union).
If I’m not mistaken, Uyghurs (a Turkic community) are found in China, Russia, Turkey, Kazakhstan and Uzbekistan which again suggests China geographically, ethnically and culturally overlaps with Russia, the Middle East and Central Asia. There are even Chinese and Turks in Russia. Not that Japan and Korea lacked any long-standing ethnic minorities. But I only know of Japan having the Ainu and Okinawa folks.
Not that South Korea never had ethnic minorities but most of them are recent immigrants. Not to mention both China and South Korea have considerable Christian populations though it’s around 2% in the former. However when counted with Muslims, the number of monotheists in China’s up to 2.98 or nearly 3% (constant intrusion).
In South Korea, it’s up to 27.8 (though the Muslim population’s small). It’s even more miniscule in Japan and I frankly know little about Mongolia (though it might have enough peculiarities to justify its differences from China). Not to mention parts of China (most notably Macau and Hong Kong), Mongolia, Vietnam and South Korea are considerably more Westernised.
China’s partly influenced by German, British and Portuguese colonisation. Vietnam used to be a French colony, Mongolia’s part of the Soviet Union** and South Koreans swore allegiance to America. Not that Japan’s any less Westernised but that the former three were Westernised to any degree due to most colonisation and geographic proximity.
When there are differences, they’re bound to occur.
*Same for Turkey and possibly both Indonesia and Malaysia
**Currently around 17% of Mongolians and 12% of Turkmeni use Russian whilst Russia’s the official language in Kazakhstan.