The phenomenon you're describing is a common observation with microwave ovens and is a result of how microwaves generate and distribute heat.
When a microwave oven is operating, it uses microwave radiation to excite water molecules and other polarized molecules within the food. This agitation of molecules generates heat, which is then conducted through the food, heating it evenly and quickly.
While the microwave is running, the heat generated by the microwave radiation is absorbed primarily by the food and the turntable (if present), which rotates to ensure even heating. The oven's design and the presence of a fan help dissipate excess heat from the cooking area and prevent the microwave's interior from overheating.
However, when the microwave is not running, the heat generated by the electronic components (e.g., magnetron, power supply) and the cooling fan during previous cooking cycles may still be present within the microwave's enclosure. This residual heat can lead to the interior of the microwave feeling warm even though it's not currently cooking any food.
The exterior of the microwave, including the control panel and outer casing, is designed to remain relatively cool to the touch during operation. Microwave ovens have protective layers, insulation, and ventilation systems that prevent the external surface from becoming excessively hot while cooking.
In summary, the inside of a microwave may get warm when it's not running due to residual heat from previous cooking cycles. During operation, the heat generated is efficiently used to cook the food, and the microwave's design ensures that the exterior remains cool to the touch to prevent any accidental burns or injuries.