Delineating Korean and Japanese "looks" already seems a fool's errand if you consider that archeological evidence demonstrates close cultural and trade relationships (or alternatively: astronomically unlikely astonishing examples of parallel developments) between the two regions dating back at least to the Neolithic period - and that the current "native" population seems to only date back no farther than that period despite archeological evidence of prior populations.

Of course this all also exists in the context of Chinese history which largely hinges on what exactly you want to call "China" historically as for most of its written history there really wasn't a single unified entity.

We tend to project backwards a notion of nationhood that in the West largely only came about in the 19th century. In Europe, as a German, I find my own country to be such an obvious example to this as people from all nooks of the political spectrum will find ways to try and shoehorn the modern federal republic into an unbroken chain of history starting with the "Germanic" tribes valiantly resisting Roman rule.

In my country's specific case, the origin myth is completely nonsensical if you look at the actual historic record. The shared identity of the various tribes settling the region only existed from the outside perspective of Rome which simply referred to all foreign territories as being settled "barbarians" (because that's what the foreign languages sounded like to Romans - to put that in perspective, imagine we unironically called Asians "chingchongs").

The first entity with the word "German" in its name was the Holy Roman Empire but the words "of Germany" were only added centuries later and for the longest time the mythological warrior Hermann who "repelled" the Roman invaders by "uniting the tribes" was seen as a villain because - true to its name - the Holy Roman Empire saw itself as the successor to the Roman Empire. It literally included parts of Italy after all and was preceded by the Carolingian Empire (covering much of the same territory but more of modern France). And of course more recently we've learned that the tribes were actually more divided than unified following the conflict with Rome and that the role of Hermann may have been heavily overstated due to the fact that he was a Roman soldier and thus provided a good basis for a grandiose narrative.

You could point at the Kingdom of Germany as a historical root of German identity but there was no shared cultural identity during that period and certainly no awareness of it among its population. The common folk for most of the middle ages would have most likely only been aware of their local ruler or clergy with a faint awareness of the overarching power structures but migration through trade not withstanding separations were often as strong between neighboring villages as between modern countries.

The closest thing we get to an idea of a "German national identity" is following the conquest by Napoleon and the rise of an aristocratic/mercantile republic monarchy which provided the democratic roots for the modern republic - but even in WW1 "German" culture was heavily defined by Prussia (which covered most of German territory). Historically therefore it seems less like German nationalism was the politicalization of a shared ethnic, cultural and political identity but rather provided a framework to fabricate such an identity in its absence. Even if you ignore the absurdity of claiming a unified "German" cultural identity, the now popular notion of there being such a thing as a "German" ethnic identity flies in the face of there still being distinct native but "non-German" ethnic populations in parts of Germany despite centuries of Germanization and assimilation (notably Danish Germans in the North and Sorbs in the East).

Much like trying to draw the line where you "enter the atmosphere" of the Earth, borders are ultimately arbitrary delineations no matter how you define them and populations will move around, mix and change over time. The abstractions they help us create are likewise arbitrary and have more to do with assertions of power and control than any grander mythology used to justify them.