First, it starts with good boots which inevitably comes with trade-offs. The more support and stiffer the boot, the colder your feet are going to be… regardless of insulation. That is why at times, even in the best mountaineering boots, people lose toes. There is plenty of insulation, but the rigidity of the boot restricts circulation. For mountaineering, rigid support is essential to hold crampons, for front pointing and for those steep and rugged pitches and ridges.
On the other end of the spectrum are soft bottom mukluks or kamiks used by the Inuit or Athabaskan people. The warmest of these designs are made from smoke tanned moose hide. They feel like bedroom slippers, offering lots of room for circulation and to wiggle chilly toes. They are also extremely light, putting no added stress on legs during those long ski or snowshoe runs. The down side is that they offer little to no support, making it difficult to get good footing and also resulting in sore feet at the end of the day.
So, it is up to you and your planned activity to determine how much or how little support and insulation you will need in your footwear. No matter whether it is a stiff boot or a soft one, I layer my feet as follows:
Foot - from skin outward: thin wool liner sock, a vapor barrier liner (heavy duty plastic bag – I use maple sap bags), a medium weight, then a heavy weight insulated wool/synthetic (PrimaLoft) blend – synthetic fibers help speed up drying. The vapor barrier sock is key to keep your outer socks and liners dry.
Boot – outside to inward: a perforated mesh insole to capture snow and frost, followed by a PrimaLoft aerogel insole, then an insulated insole of synthetic felt with perforated reflective Mylar, then an insulated synthetic felt boot liner with reflective aluminum. Thicknesses of insoles and liners will depend on temperatures needs.
For mountaineering boots, I also ensure the boot has a good lace system - with beefy laces that won’t cut your fingers; I minimize use of Velcro. In extreme cold, one way to mitigate cold feet in rigid technical boots is to loosen the laces when not on technical terrain and only synch them on technical segments. I also select a boot that is one or two sizes bigger. This way you can add an extra insulted insole and extra thick wool socks - always ensuring I have plenty of space to wiggle my toes.