My dad says when he went ice-fishing with my grandfather they would put caught fish in a bag in the trunk. The fish would freeze on the drive back home but as they thawed back in their kitchen they would come back to life and start flopping around. Others have reported this, too (1). I can only guess their interiors were not frozen, and just in some sort of dormant state.

We have a small fishpond in our backyard just outside of Boston (kind of like this one, but a little deeper: https://www.pondmarket.com/epdm-pond-liner-rubber-liner-coat...). We have a few comets and shubunkin. Every winter the fish go dormant, usually around late October or early November, sinking to the bottom half-hidden by dead leaves. The top eventually ices over a few inches. We keep the small pump going and try to break the ice bubble if it freezes over, but otherwise the fish are not active for 4-5 months (with one exception, described below), until warmer weather comes in mid-April. It's an amazing thing to see them slowly reactivate and start to take food again.

One winter the pond iced over and they were still swimming around a little (2) which I can't explain. It's the only time we've seen this in the past 6 or 7 years.

1. https://biology.stackexchange.com/questions/1661/fish-coming...

2. https://www.instagram.com/p/BAR4PG8pJrn/?taken-by=ilamont