I am aware of the Ainu, though I'm certainly not an expert on the topic. Wikipedia suggests that there may be 25k to 200k Ainu people in Japan, which would be 0.02% to 0.16% of the population; and supposedly the language is nearly extinct. So my point about Japan being monoethnic "for all practical purposes" still stands, as Ainu culture is not widespread enough to make a difference.

I suspect the broader point you're trying to make is that the concept of Japan (or any other country) as a monoethnic country is in many ways manufactured by politicians and other powerful interests over time; and in many cases literally manufactured by genocide. And I grant you that's entirely true, potentially in every single "monoethnic" country. But, manufactured or not, there is a tangible difference.

I myself grew up in a monoethnic country, with the presence of a unifying cultural common ground; for example, I could rely upon the fact that a popular television show was being watched by nearly everyone in the country, even in the remote corners. And that's just one minor example of what adds up to the sense that yes, this stranger next to me has had a similar life to me, at least in what we were taught in school, the media we consumed, the food we grew up eating, the behavioral customs we expect of each other, and so on.

The absence of this unifying cultural common ground is the characteristic of a polyethnic country.

> that there may be 25k to 200k Ainu people in Japan

My point is that the Ainu people were... not consulted on whether that is a good number to have or not. You're seeing effect, not cause. To put it very, very delicately.

> I myself grew up in a monoethnic country, with the presence of a unifying cultural common ground; for example, I could rely upon the fact that a popular television show was being watched by nearly everyone in the country, even in the remote corners. And that's just one minor example of what adds up to the sense that yes, this stranger next to me has had a similar life to me, at least in what we were taught in school, the media we consumed, the food we grew up eating, the behavioral customs we expect of each other, and so on.

This description of poly-ethnicity applies to Japan more than I think you realize.

Food in the southern prefectures is pretty different from food in the northern prefectures. Particularly Hokkaido and Okinawa have very different food cultures than a lot of places on Honshu. Likewise the architecture and structure of towns, cities, etc varies pretty significantly. Okinawa is practically a concrete jungle and Hokkaido has a lot of red brick buildings that wouldn't be out of place in Russia or Europe. Cities in both prefectures are much more car centric than in cities of comparable densities in the other prefectures.

Even just on Honshu you have things like which side of the road/sidewalk you walk on or which side is the up vs down stairs being different by which prefecture you are in. Likewise, there are pretty significant differences in cultural customs and dialects between prefectures. A word or phrase will have a fairly substantial weight behind it in one prefecture but simply be light/playful in another, or vice versa.

But then the media being largely the same regardless of where you are in the country is pretty similar to the US.

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Point being that there is a lot of cultural variance in Japan that gets reduced to a perception of a monolithic culture even within the regions that are predominantly composed of Yamato Japanese people.

But when you start looking at regions like Okinawa, Hokkaido, Kagoshima, or the Izu-Ogasawara islands, you realize that they have far too different of cultures to be assumed to be minor variations of one mono-ethnic culture.