1. China and Taiwan have had several skirmishes since the PRC came into being in 1949 -- there just haven't been any in recent times since China doesn't want to risk American intervention or accidentally hitting a US Navy ship in the Taiwan Strait. The status quo is China effectively allows Taiwan to function autonomously so long as they don't claim outright sovereignty, which is when China starts flexing the warhammer. The current government in Taiwan has floated the idea of declaring independence far more openly than in the past which combined with Trump's wishy-washiness on alliances raises the risk that China might try something, even if not outright invasion.
2. Japan committed some horrific atrocities they have yet to atone for and Shinzo Abe is certainly not helping the situation with his nationalist rhetoric either, but China is chomping at the bit to justify a conflict with Japan (see the Diaoyu/Senkaku island situation). I take it you're familiar with Korean history and current events, so it's effectively the whole comfort women dispute x10.
3. What the rest of the world sees of Chinese social media is likely a filtered version, and some platforms like Tik Tok straight up have different versions for China/the rest of the world. Who knows, perhaps the excessive rhetoric there is just users trying to buff up their social credit score, but the fact that there are different versions for Chinese users and external users feeds the divide (probably by design).
4. Nobody can know the full extent of what goes on there given how deep Chinese surveillance runs, but we can draw a negative inference from that itself. Why does Xinjiang need special surveillance programs and restricted access even beyond the measures already imposed in the rest of China?
I get the comparisons can be a touchy subject but I think the world ought to have its moment of reckoning with China sooner rather than later, since later might turn out to be too late.