You’re not supposed to defrost meats in warm water due to harmful bacteria growth, you’re not supposed to leave food at room temperature for long without being refrigerated because of harmful growth but, when you go to a buffet, food can sit out for hours under heat lamps and be fine?
In: Biology
There’s a concept in food safety called the “danger zone”. It extends from about 40 F / 5 C to about 140 F / 60 C. You can keep food below the lower end of that range, or above the upper end, for a while without it going bad. The reason is that this is approximately the range of temperatures that the bacteria that cause foodborne illness can tolerate well enough to grow. They can survive in lower temps, but they won’t grow or will grow very slowly, which is why you can store meat in the fridge for a few days without problems. (They’ll die at higher temps if they’re kept that hot for a while, as in sous vide cooking, but can survive brief exposure.)
The food at the buffet is (at least supposed to be) kept above 140 F / 60 C, so that whatever bacteria may be present can’t grow effectively. The trays at a buffet usually sit above a pool of very hot water in addition to the heat lamps, which keeps them hot. (This is if the food does stay out for hours. It may simply be changed out regularly, in which case it can be kept in the danger zone.)
You can take the “keep it above the upper end of the range” principle really far, because temps above 140 F / 60 C will kill almost all bacteria with prolonged exposure. This is the principle behind [perpetual stews](https://en.wikipedia.org/wiki/Perpetual_stew), foods which were effectively cooked for years or decades.
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