I live in a pretty hot climate, but it’s only really unbearable at times due to shoddy building and bad urban planning. Even then, summer can be difficult.
I can’t imagine what it’s like on the equator, especially in dense urban centres. What’s Mumbai, Bangkok, or Singapore like at the height of the wet season?! How do millions of people function day to day?
Expose yourself to moderate heat as much as possible. Your body adapts and then you can handle the extreme heat better.
A lot of people hide inside with AC all the time and then it’s a shock to the system when they go outside.
Also, body composition and fitness can make a huge difference.