In mainland China, the only examples I've heard of were leading academics. The standard is "outstanding contributions" to China. That's what I mean by "very high bar." Next to impossible is not a bad way to describe it, but next to impossible isn't the same as impossible. Thousands have done it. That's not a lot. I don't think it's even tens of thousands.

On paper, there is also path by which you can marry into Chinese citizenship, but it's exceptionally rare in practice.

"Renounce other citizenships" is complex. Many nations don't provide a citizens with a means to stop being citizen. Which is to say, if you take a Chinese (or any other) citizenship, and renounce your old citizenship, you may still end up with both citizenships.

I don't know specifics about Honk Kong, but I do know the standards are laxer than mainland China.