Constructing a down jacket is difficult, and we have been perfecting ours for decades. There are two main types of construction. Most down jackets are stitched-through, where the inside and the outside of the jacket are stitched together to make channels which the down insulation goes in. This is ideal for lightweight jackets but for very warm jackets it creates too many cold spots, and so a box-wall is used, where the inner and outer are kept separate, resulting in a warmer jacket. Box-wall jackets are heavier than stitch-through jackets, but if you want an extremely warm jacket then they tend to be the best option.
There are various other ways of making down jackets too. We developed our Firestorm construction over the course of three years of tests. Essentially a whole separate layer is put on top of an existing down jacket construction, which increases wind resistance, warmth, and water resistance, while adding very little weight or bulk. You can read more about our Firestorm construction here (link to the blog post we did). Another construction unique to Mountain Equipment is our patent-pending Dual Offset construction. This has the low bulk and excellent comfort of a stitch-through jacket but without the seams that reduce wind resistance and warmth. For many uses, this provides a benefit over both box-wall and stitch-through constructions.