Living in Austria we have very strict building regulations and a lot of insulation and despite of this there are more and more air conditioners because it’s so hot outside in summers now that even nights are too hot. No insulation in the world can fix that, especially if heat is also created inside.

Spaniard here. While not ubiquitous, household AC is not even rare nowadays.

It depends a lot on the area though: it's uncommon in the north/west (where temperatures get pretty low during the night even during the summer), but not in the south/east where temps won't go below 25ºC for several days (and nights) in a row.

It's possible, but not well-advised. Heat deaths in areas with poor AC penetration are, without fail, higher than in those with AC deployed.

but i would wager that they do not live in perfect conditions inside those houses