Depending on what their church district allows, those buggies have battery operated heaters in them.
They can also heat bricks and take them along.
One very strict group in Ohio does not allow ‘topped’ buggies, which means the people are in the open regardless of the weather.
There are dozens of people who make their living driving the Amish. Their white 15 passenger vans are too numerous to count in Holmes county OH. They are quite busy when it’s cold.
There were a lot of Amish at Assateague Island this summer. They are in full garb, but they have the whole enclave with them in vans. They cook on the beach in designated areas and stay the whole day. It was the first time I’d seen them in such numbers and so often.