what is the difference between a fridge compressor and freezer compressor and is the answer the thermostat?

There is a difference. A fridge compressor will have a bigger electric motor inside the compressor housing. A freezer works at lower temperature, which means less pressure which means the motor doesn't work as hard. I know it sounds backwards, but that is the way it is. There really is s difference between compressors, and it really is important to use the right one.

the answer is the thermostat,PatF85 what is the air flow door

What are you asking ?? Household refrigerator/freezers have only one compressor. The evaporator coil is in the freezer along with a fan. Which blows cold air throughout the whole unit. The cold control is the 'thermostat' for the freezer, and an air passage way with an adjustable door, regulates the fridge temp. So the freezer temp knob regulates the compressor when to kick on & off, and the fridge temp knob regulates the temperature by adjusting the air flow door.