I know that they do flash freezing to kill parasites. But flash freezing only kills parasites, not bacteria. How they kill salmonella and stuff?
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It should be noted that people often have misconception that all bacteria is dangerous and as such everything has to be sanitized.
Only some bacteria are harmful to humans and they can be so in few different ways. The most obvious way is that bacteria can infect humans through digestive track. In such cases the food has to always be cooked to kill even trace amount of bacteria (salmonella is good example) Fish don’t have these kind of bacteria.
Some bacteria can cause dangerous toxic by products (Botulism for example) And these cases it’s important not to give bacteria time to accumulate these waste products. So when stored properly food is edible, but if stored improperly or for too long the food become in edible. Botulism is extream example since it can be deadly but numerous other bacteria just cause diarrhea (which is body way to avoid digesting dangerous toxins) This is actually what refrigeration is for, preventing bacteria from growing too much and accumulate toxins.
Some bacteria produce safe by products. These can actually be used to store food, since other bacteria can’t use the by products of healthy bacteria. These bacteria can be used to produce cheese and actually store food to extend it’s self life.
When we store thing in fridge we slow down the growth of bacteria, but if there was no bacteria to grow freezing wouldn’t do a thing. In fact the food where we have entirely eliminated presence of bacteria can usually be stored in room temperature (like cans) or bacteria is incapable of growing on them (flour for example, unless wet).
To come back to Sushi. Fish generally don’t have bacteria capable of infecting humans, so we are mostly wanting to make sure there isn’t too much time for bacteria to accumulate toxins but that’s why sushi has to be fresh. Additionally the fishes meat is actually relatively bacteria free, though some bacteria can get on the fish from the enviroment or the gut of the fish. Part of the process of preparing the sushi fish is to make sure as little of bacteria has opportunity to get on the meat as possible.
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