>I now believe China’s actual population may be as low as 300–400 million

that we now live in a world where people are confident enough to make claims this stupid in front of a camera should frighten anyone.

Some basic logic, if China had the population of the United States it would have magically acquired the per capita economic output of the US in ~30 years, consume several times the energy and food it imports and somehow have produced several cities the size of Tokyo. The fact that China produces ~50% of the world's ships and has the manufacturing output of of the G7 combined is impressive with over a billion people, but hey they must have some space age technology to do it with 3% of the world's population!

In philosophy there's a concept called the coherence theory of truth, if you want to know if something is true check if it doesn't defy basic logic or other facts you know, great tool instead of believing what youtubers say

Youtube videos are always a poor quality source - the UN doesn’t accept China’s numbers exactly but they believe the total number is broadly correct due to cross referenced data, and expert independent demographers largely agree. The figure of 1.4 billion is likely within the ballpark and the idea that this is off by hundreds of millions is considered a fairly fringe theory, almost a conspiracy theory.