Airliners at cruising height (FL300-FL410) normally report outside temps in the -50C to -60C range. -56C seems to be the most common one. I doubt a Shackleton bothered those altitudes very often though :-)
Haven't personally seen anything colder than -60C, but not saying it doesn't happen.
The Tristar etc at BA easily, and regularly flew over 30,000
Once again, I'm no expert, but surely temperature at altitude will depend on which part of the world you're in.
This is also true. Lots of variables and the aircraft have to reliably perform in any conditions that they might encounter.
Fuselages are heavily insulated to help stabilise internal temperatures.
Avionics hot air gets expelled into the forward hold and/or out of a vent. This depends on altitude and cabin pressure.
Bleed air from the engines/apu/external source is used to pressurise the cabin via the AC packs which regulate the chosen temperature. The pressure is regulated by a flap at the rear of the aircraft.
This system can keep an A340 cabin at 25°C on the gound in Antarctica with a door open and at the other extreme at about 15° on the ground in Dubai.
Without any bleed air, and doors closed, the cabin temps can easily hit 35-40° on the apron at Gatwick at this time of year. Doors open with a slight breeze running through the cabin takes the edge off.
With all the electronics powered up, even with the flight deck windows open, it's not uncommon for the flight deck temps to be north of 30° in the hangar. Again, no bleed air.