My summers growing up were full of camp. Cub Scouts starting when I was six, Boy Scout camp every year after that, and plenty of other camps in between, week after week spent outside in the worst heat of the year. That is most of my childhood, really, learning to beat the heat and live in weather that settles in and does not let up. Do enough summers like that and you stop fighting the heat and start working with it. You learn the tricks, and most of them are simpler than people expect.
This is not medical advice, and I am not going to pretend it is. If someone is genuinely in trouble out there, the move is to call 911 or your local emergency services and get real help on the way. What follows is just the practical stuff, the habits that keep an ordinary hot day from turning into a miserable one.