A jacket works by using your body heat to warm you. the jacket acts as a trap to keep in warmth.
it is filled with all that radiation and keeps the warmth in it
Feathers, like most forms of thermal insulation, work by trapping a layer of air. Air is a good thermal insulator, provided that it does not circulate.
Feathers are efficient structures for trapping air. Motionless air is a very good thermal insulator.
It insulates the heat of your body temperature in the bag.