As I posted already, they do not have to actually make them all.
Chinese CAN make quality (just look at their military tech if in doubt).
So, provided a customer ensures strict control of material specification, and design, and has strong QC processes in place, there is no reason they could not get quality fasteners from a Chinese supplier, but ALL of the aforementioned must be in place.
But buying on line as a private person from unknown sources is a VERY different matter, you have none of the aforementioned processes, and no ‘leverage’. If / when allowed, the Chinese are masters at making things to the lowest cost and not caring one jot about quality, or even being fit for purpose.
A well known trick of Chinese suppliers is to provide the first small batch to perfect standards. Then, when trust is established, and a large order placed, that’s when the crap starts being shipped !
Even big firms in the medical industry have been bitten by certified high grade Chinese supplied stainless steel being of the wrong grade, all the certificates and official stamps and signatures ALL fake…!