Down jackets extreme cold

A cold-weather down jacket is designed to cope with very low temperatures, thanks to denser padding and reinforced technical finishes.
It provides better insulation and protection from the wind and intense cold, where a conventional down jacket would reach its limits.

This depends on the filling (natural or synthetic down), the weight and the thermal construction.
Some down jackets can protect against temperatures as low as -15°C or -25°C, depending on the model, provided they are worn with appropriate layers.

Yes, as long as it's breathable, light and windproof.
For prolonged efforts, it can be used as a static layer (break, bivouac, ski lift) or as an outer layer after the effort.

Wash at 30°C in a delicate cycle, using a special down or technical detergent, without fabric softener.
Dry slowly and gently (ideally in a low-temperature tumble dryer with tennis balls) to preserve loft and insulation.

They are often warmer for the same weight, and more comfortable to wear thanks to their wrap-around padding.
On the other hand, a parka is generally more waterproof, and a multi-layer technical jacket offers modularity better suited to intense activities.