Would someone with more fat stay warmer than someone with a lot of muscle?

someone with a lot of muscle

The muscular person produces more body warmth. So the muscular body would be warmer, as long as they are well sustained and have energy to burn.

The fat person has got better insulation. So the fat would keep them warm like an extra set of clothes.

In a situation where two such people are lost in the wilderness in wintertime with insufficient clothes and no supplies, the muscular person would get along better at first. Because he/she would burn the stored energy in his body and produce body warmth with that.

But as soon as the two start being hungry and their bodies run out of energy to burn, the fat person would be better off, as he/she is sort of his/her own insulation. The fat would keep the core warm. The muscular person would die of the cold sooner.

Fat is an insulator. It does help keep people warm. Muscle is not as good an insulator but it requires more calories to maintain so it generates more of its own heat. A fat person is going to stay warmer than a muscled one. Fat people do not loose heat as quickly as leaner folks. In the cold skinny people start shivering before fat people do. Conversely fat people have more trouble getting rid of excess body heat when they are hot.