China's food supply chain has a lot of inefficiencies spread over many small farmsteads with old technology and poor conditions instead of highly mechanized large agriculture farms in the west. I don't remember the exact numbers but lost in harvest, storage, transportation, refining was equivalent to total food imports before it reaches consumers. There's also massive waste in food services, the number is something ridiculous like enough to feed 400 million annually. This was from a study from ~5 years ago.

What makes rice poisonous, and how is it made safe for use in snacks?

Possibly referring to this: https://en.wikipedia.org/wiki/Rice#Bacillus_cereus

Rice with heavy metal such as cadmium, it's pretty common so many don't even test it, those that are unfortunate to be tested can still be sold to rice snack factories.