It is not a guarantee but fire used in combo with a decent shelter is the best you can do. Shelter is by far the most misunderstood skill set. Shelter will not keep you alive if it is cold enough, only external heat or heat maintenance.
I agree completely that a shelter/fire combo is best.
Your body heat in an enclosed shelter that is just big enough for you to get in--that insulates you from the ground and the air outside--creates a microclimate.
You lose heat in a combination of five main ways: from your body to a cooler object, from your body to cooler moving air, from your body to the ambient air, sweating and breathing.
Something that keeps you off the ground, reflects your body's heat back in on it and blocks the cooler temperature will serve you better than a fire that only warms one side of you, that you may not get started or have enough wood to maintain.
A combination of both is best, yes, but if you cannot have both; the COMPLETELY enclosed, close fitting, insulated shelter is your best choice (i.e. a-frame instead of lean-to or tent).
If you already have drop in core temp a secondary heat source is needed to get it back, fire among other things helps with this, shelter does not.
Here's where we disagree. As I'd said: microclimate. Your body doesn't lose temperature and then shut down all at once. Hypothermia comes in stages. Your body makes its own heat and is able to replenish it. Hypothermia is the result of your body not being able to maintain enough heat to keep up with what the environmental factors are wicking away.
Your body can withstand a certain amount of heat loss without shutdown. You'll be shaking and shivering (possibly violently so, depending on how bad), but that's also your body's way of contracting and extending muscles--a defense mechanism to try and get warm. Physical activity of any kind will aid this process.
Fire requires loads and loads of fuel to maintain, only warms one side of you at a time and can go out if it decides to rain at night. And then there's no guarantee that you will even get one started if the wood is too damp or your lighting method doesn't work.
A secondary heat source would be excellent. But your body does not depend on external secondary heat sources for survival, if it did you'd be a reptile and not a mammal. The heat your body produces is due to the internal processes. The simple act of building the shelter will aid these processes, once the shelter is built, climb inside and close off the opening. The roof and sides will reflect your body's heat, the 4 to 6 inches of pine boughs and debris on the floor will insulate your body. 360 degree protection.
I'm not talking about a tarp with three open sides. That's a rain shelter and not good for much else. Well built shelters from the cold and environment factors close in on all sides and are thickly insulated. These can range from a snow cave insulated with debris and pine boughs, to wikiups with insulated floors.
As I am getting dressed to go out into -43 this very minute, the reality of cold hits you squarely in the face. Some have to be here to get it.
Skam
Exactly. Your clothing is the first line of insulation. Dress for the weather. You'll have enough layers of clothing on at that temperature that you should be able to shed the damp ones and still be warm during physical activity. Which will hold in enough heat to allow you to get into your quarters and warm them.
ETA: Clearly the choice between fire or shelter depends on circumstances and factors (like everything else in a true survival situation): how wet you are, time, temp, weather...everything. One will always be a better choice than another. If I were soaked to the bone and there was no chance of rain that night, you can bet your butt I'd find a place to build a good reflector fire and warm me up some pine needle tea, then build a fire bed on the coals if I could.
Enough weather will put out any fire.