The reptiles have approximately the same temperature as the ground beneath them. It is heated by the sun and will therefore have a higher temperature than the air.
When the temperature gets low, reptiles can flatten their bodies so that they can absorb as much heat from the ground as possible, while at the same time gaining heat from- the sun from above. The larger the surface, the more heat.
The heated blood is transported to the colder internal organs to achieve a certain core temperature. When it gets too hot, the reptiles will leave the warm place for a cooler place or shade. In some cases, lizards can raise their bodies off the ground, allowing air to pass underneath them and lower their body temperature.