I haven’t played since this was added, so I don’t know what to do.
Either A - using a pot/frying pan to “heat something up” through the action menu, or, more slowly, by simply having the items in an enclosed space with a fire for a few minutes.
There are also heat packs to do the job, but they are single use, just so you know.
If you have vehicle storage tanks full of water, you can still use them to make tea and stuff, or make clean water and put it into a thawable container for later.
The thermos is useful. You can find them in trash cans and such.
Thanks for all that info!