The magic in this movie resides not only in McDormand's performance. It rests also in her fellow 'actors'. Almost all of them are not acting. They are truly nomads. They live on the road. They are part of a mobile community that travels from place to place, a kind of underground society of people who, for one reason or another, have taken up anchor and have decided to keep moving.
To be sure, the motivation for all of them seems to be essentially economic. The first stop Fern makes is at a campground near an Amazon warehouse. She arrives at night and the next morning goes to work at the warehouse. It's Christmas. Amazon needs seasonal workers.

Dear @agmoore !
I thought Americans living in trailers were the poorest class in America! This is the first time I've seen them wandering around as seasonal workers!

Do you want to be nomad?