How can people without electricity heat their homes in winter?

Heat comes from certain conditions. Friction, chemical reactions, and combustion. Our bodies are an example of chemical reactions causing heat. The easiest is combustion (fire). Rubbing two sticks together causes heat, but that heat can also cause combustion which will turn matter into energy.

Heat requires a fuel. So the true answer to your question really depends on what fuel is available. Sunlight, wood, fossil fuels, chemicals, and most non dense materials can be considered fuel. Fuel is merely energy stored in one form, then converted to a more usable form.