Thermal bags work by using a reflective material (often aluminum) that reflects heat radiation, stopping it from escaping. They also use an insulating material, usually plastic, paper or foam, to stop heat escaping through conduction. Combined they trap a lot of heat allowing your food to stay hotter for longer.
What is inside a cooler bag?
Instead, they are usually made from heavy but flexible fabrics, like polyester, on the outside. The inside is lined with heavy duty foil. In between the outer and inner layers are layers of materials like flexible foam, which are thin but dense and have the ability to maintain internal temperatures for several hours.