They aren't really. 98% of them are Japanese citizens but that doesn't mean they're "Japanese people" - the government doesn't track that.
Also, the concept of "Japanese people" was invented in the Meiji era as part of their approach to resisting Western colonization by pretending to already be a Western nation-state.
Also, the concept of "Japanese people" was invented in the Meiji era as part of their approach to resisting Western colonization by pretending to already be a Western nation-state.
And of course, some of them are Ainu or Ryukyuan.