It's not that cold blooded animals produce no heat, it's that they lose it faster than they make it. Plus a larger mass retains heat for longer.

Cold blooded animals still produce body heat, just like you feel warmer after exercise. They may not have any methods to keep their body temperature much higher than the surroundings, but wouldn't want to become too cold either.

Edit:

While many snakes hibernate in solitude, there are some species who hibernate in groups. This is so they can preserve warmth in their dens through the winter.

https://faunafacts.com/do-snakes-cuddle/

'Cold-blooded' is a term that's a little outdated, because of the confusion it creates around this very concept. https://en.wikipedia.org/wiki/Cold-blooded has some information on it, and even says it's an informal term. Generally speaking, if we refer to something as cold-blooded, it's more reliant on or affected by the external temperature, while warm-blooded creatures will 'deal with' the external temperature.

A larger mass loses less thermal energy because it has less surface. So the snakes bask during the day and store up heat, and then huddle together to keep that heat around for longer. Same as a big rock would hold heat for longer than separate tiny pebbles would