If you have enough, they'll huddle up in the coop and be fine. There's some discussion around how many in what size coop is necessary to ensure that. And, there's still the necessity of the coop being adequately made so that there's air flow, but not so much as to make it difficult for their own heat to work.
But chickens are little furnaces. You give them enough food, and access to water (when it's cold they need water as much or more than food), and they can keep a coop well above freezing with little trouble in all but the worst weather.
We have so few birds that we can't keep them outside in freezing weather, so I've forgotten all the stuff that goes into making sure they're going to be in the right situation, but that info is out there.
For the most part, it's iffy to heat a coop because of fire risks, or that's the info I got.
I will say this though. We're up in the mountains a bit, so it gets cold. One of our birds is half feral and refuses to stay in the coop at all. She sleeps in a tree, or in a shrubbery (ni), and has not run into any trouble yet. We're working on getting her tame enough to at least share space with the rooster on the porch, but it's slow going.
But some of the neighbors have lost birds to cold in the past, even in coops; and the feral flock tends to fluctuate down in size a lot more in winter than summer.
For real though, ours are pets, and rather pampered overall. When they're cuddled up with one of us, we can break a sweat from their heat.