..?
That's either true or false of both the United States and China depending on your definition of "entire group of people". If you want to say one does it, then the other does it. If you want to say one doesn't, then the other doesn't.
What is the US doing to a group within its own borders that's equivalent to the Ughyrs?
Is it 'moral high ground' when you just haven't managed to catch people like Edward Snowden or were denied the extradition of Julian Assange?
I'm not denying the issues with Snowden or Assange, but that pales to what China is doing to dissidents within its borders.
We at least know where Snowden and Assange are. The same can't be said for the people within China who've simply disappeared.
Anyway, Chelsea Manning, various Black Panther Party members still in prison, Water protectors, BLM activists jailed on ridiculous charges, and so on.
Beijing is a trend setter on a number of issues, but this one is nothing to be proud of.
freedomhouse.org
Again, all true, but it's not equivalent in scale.
The US hasn't imprisoned journalists for speaking out against the government.
Abu Ghraib? Guantanamo Bay? Biden is currently blocking photography of ICE detention centers.
en.wikipedia.org
However one feels about Abu Ghraib and Guantanamo, those were used for enemy combatants.
Similarly, I don't think ICE can be put on the same level as everything that China is doing. One is used to apprehend illegal migrants, the other is to detain political prisoners.
The United States regime seems to murder far more people before they would even make it to a prison and supports military coups (Bolivia, Haiti) and genocides (Yemen) and brutal apartheid/ethnic cleansing (Israel), as well as committing heaps of structural (economic) violence against its poorer residents, so I agree: it's hard to cast it as morally equivalent. It appears to be worse.
Everything you've said is more or less true, but if we're doing this...
Bolivia and Haiti? Well China supports North Korea and Myanmar.
Genocide in Yemen? Again, true, US arms sales have played a role, but China is also a major arms exporter.
US is supporting Israel and ethnic cleansing? China is doing that itself in Tibet.
Economic violence? Look what China's done to its own environment and people, not to mention the use of slave labour.
To be clear, the sins of the US don't excuse the sins of China and vice versa. However, the original question was that of detention. So on that note, which of the two is the worst offender? China, or the US?
To me, China. By almost any measurement, be it democracy, freedom of the press, freedom of speech, or the global slavery index, the China falls below the US. That's arguably not saying much, but one's clearly doing better than the other.