Iwatched a video and they froze a fish for a week and then put it back in water and let it thaw. and it came back alive and started swimming. how is that even scientifically possible? ?

The fish was frozen so quickly that none of its internal organs/vital structures were able to die before freezing, nothing then damaged said body parts, so the fish is in stasis, no bodily functions are proceeding.

Due to the fish being so small, it is able to warm up at a faster rate than any process that kills it, such as bacterial eating it. It gets to a point where blood can start circulating again, once the heart is warm enough to continue pumping, because the cells weren't killed.

My chem teacher said (during a tangent) that it is theoretically possible with humans but it takes too long to warm up the entirety of the body, due to it being a good insulator of heat.(this is not my own words i found it on a science website)