> how did people live before ACs were invented? can't imagine

There's the part where people were more resistant to heat one way or another (either they were more used to it, or they made adjustments and had better coping mechanisms, or both).

The other part is the gradual change in climate and environement where it might have gotten way worse than before. For instance having houses with bigger windows/worse insulation, more pollution pushing people to close their windows, more asphalt/less heat dissipation, more humidity and sheer temperatures raising.