I used to live in Mexicali where IIRC we reached 53C one summer for a few days, the rest were max 50C for a couple of days (during the time I lived there), there was no rain, no clouds, and little to no wind.
I had an office job, so only my commute had me outside during the day, maybe also going to buy lunch.
For the most part it was only to go out after the sun had set if I wanted to do something. All the stores, houses, and businesses have AC, also the public transit even if it just mildly lowered the temperature.
I heard stories that before AC people just endured it, sitting outside at the shadows of few trees and other kind of covers.
Also the original settlers made an underground city to endure the heat.
IIRC the first mall that had AC (La cachanilla) was so popular that during summer it was common to be incredibly full, the verb “cachanillear” was used to mean to go walk in the mall, mostly just during the summer.

Now in Montreal the heat is very humid and I miss the dry heat of the dessert haha.
In here the houses are built to trap heat for the winter, but it backfires during the summer.
Again, with an office job, and now fully remote thanks to the pandemic, a portable AC does the trick.

Also, I don’t remember where I read it, but simple fans stop being useful after certain temperature, but I don’t remember what it was.

AFAIK is every community your instance has access to that has already been accessed by someone in your instance.
What I mean is if someone creates a community in lemmy.one it won’t show up for anyone in lemmy.ml until someone manually searches for it or for a post in that community.

It’s not possible https://github.com/LemmyNet/lemmy/issues/1985

The other issue linked at the end says the possible solution is to export/import some data, like subscriptions. But it’s not implemented yet.