I have never met someone raised outside of Asia that has ordered a glass of hot water to drink as-is. I have no idea why this habit is so wide spread among people raised in Asia and it baffles me.
Before sanitation rules, very broadly: Europe made alcohol to make potable water whereas Asia boiled it and made tea. When there’s no tea available or fitting your tastes, the water still needs to be purified, so drinking hot water was still a common practice which has stayed around as an aspect of culture.
In Europe, to make drinks like beer you had to boil the mash, which unknown to them sterilized the water, which made beer generally safe to drink.
In east asia, as you mentioned tea was a common drink. But before that there were numerous herbal remedies that had to be boiled and served hot as well. People who drank the herbal remedies got better (mainly because hydration and clean drinking water are important factors for well being). Other than attributing the recovery to just the herbs, they also attributed it to the temperature.
So lacking tea or herbal drinks, the ancient chinese believed drinking hot water was somehow beneficial to the body. Add that to the fact that many who drank cold untreated water fell sick, you can easily see how the myth developed.
Another side note. Hot water is expensive (fuel wise) so drinking hot water was a sign your family was comparatively well to do and something a lot of villagers emulated in an attempt to show that the family was well off.
I know of two ways it can be. You know how the body fights infection with a fever - well I have some chronic inflammation in the gut that is exacerbated by cold exposure, and the gut becomes more leaky after going out in the winter. Tyramine from aged foods leaks into the bloodstream, causing various symptoms. I drink hot water now.
I haven't heard of it, but I guess it makes sense. Like, it's not uncommon in the US to drink hot coffee in the morning when it's cold out if you're camping or in an outdoors environment that's hard to heat up. Delivers a big slug of heat directly to someone. But there's no real reason that it has to contain caffeine.
I don't know about Korea or other places, but Japan traditionally didn't go in for house insulation, aimed to use the kotatsu rather than heating the living space as a whole.
I love that kotatsu idea! That would be such a cozy way to rest in on a cold weekend. This should be a thing everywhere …. But only the electric version. I’m not sticking my feet under a mystery blanket with a charcoal burner somewhere