> Why would they waste time and their own money researching things when they can just steal it and then give that information to private and public Chinese industries?

Because this way you'll always be behind.

Or you do both. Start from the point of known technologies and reverse engineering and improve from there. China bootstrapped their high-tech industry with not just spying, but also licensing technology, join-ventures, training students/employees abroad and then bringing them back as experienced researchers/engineers. No need to start from first-principles. Plus even when you do have tech-specs on something, that doesn't mean you know how to manufacture it, or it may need modification to address broader system tradeoffs.

If you start from behind and are trying to execute from first principals you'll usually lag behind too just because you have to relearn all the things countries like the US did on your own.

Well or by capturing nazi scientists

I guess the modern equivalent of that would be espionage which afaik China has not been shy about executing. Operation Paperclip was somewhat of a fluke historically speaking for the explosion of nation-less (ish) scientists. The next one in rocketry would have been the fall of the USSR I guess.