afellowkid answered for the Ainu in the North, but I’ll add to that the Ryukyuans in the South.

While (I think) they are content to be a part of the Japanese nation, their language and culture is markedly distinct, to the point that the government’s decision to not recognise them as a distinct group is absurd by any standard - it’s as stupid as when France would insist ‘Algiers is France’, and only gets further mileage in the anglosphere because unlike the Algerians, Ryukyuans and Ainu get tarred with the insipid Western ‘looks the same = is the same’ thought.

What I mean by “And the lens through which you have to view Japan to think it’s homogenous in any respect is pretty telling” is that for someone to claim Japan is a monoculture means they’d have to either be parroting the government’s official stance (which is a deliberate effort to erase those cultures), or they have no idea what they’re talking about. Or both, in that their entire conception of Japan is constructed by exported Japanese media that largely sticks to the monoculture narrative.