Basically their mechanical moving parts are much more simplified, their landing gear is stronger, their engines require less maintenance, and the planes are made almost self servable. For example, the Il-86 has a little door in the cargo hold where the passengers could put their luggage in instead of airport service doing it. The Yak-40 is an ultimate plane for those Siberia airports because it is very simple, the flaps are one slot, it has 3 engines "just it case", an automatic opening ladder in the back, low pressure tires and many more features to make it as less complex as possible. If you haven't noticed, many Russian planes have lots of tires compared to their western counterparts so they can land on rough runways, and their systems are triple reserved so that if one fails the plane can still make it out of the airport to a more civilized one. Or for example the wheel base of the Tu-134 and Tu-154 is very wide for better stability on the snowy runways. However some of these things lead to weight increase and increased fuel consumption which wasn't much of a problem in the Soviet Union. As for cold weather all planes are designed to withstand cold not just in Russia.