Which mechanism allows this in China?

Voting. The main difference is that the division line between parties and government is placed differently than in democracies. There are advantages and disadvantages of course, but the main point is: it’s silly to believe there’s no social feedback mechanisms in a country changing as fast as China does.

It's a one party state? Operating under omnipresent censorship?

There certainly are street protests, usually in response to local incidents, but the whole transmission mechanism is extremely limited to the bare minimum. And certainly not capable of discussing privacy in any Western sense.

Because the division line is shifted, the term "party" means something entirely different. So, yes, in one party state there are still groups within it, with different beliefs about things, and they work toward consensus using voting. Street protests are just one of the forms of democracy (and they are often organised by party).

As for privacy in the western sense - does this include USA, with no personal data protection at all?

No dude Chinese people can't protest they are all stupid drones... /s

Replace "stupid drones" with "thrown in jail, families abused, disappeared" and you have an accurate, non-sarcastic assessment.

Look to our friends in Hong Kong.