Why does the cooling fan come on immediately on full and stay on as soon as I start the car

Do you have the defroster on?

No, and the thermostat and the cooling fan unit has also been changed in the last week. I thought it was fixed and then I started the car 2 days later and the fan came on immediately and was very loud.

There are two sending units for coolant temperature, one is for the fans and another for the gauge. In this image I drew a red arrow to the one for the gauge. I could not find an image pointing to the one for fans. When standing at the front of the car looking at the motor, the one for the fan is just to the right (pointing upwards) of where the top radiator hose goes into the motor on the intake manifold. To the left and lower on the intake manifold is the sensor for the temp gauge. When you say 'cooling fan unit. do you mean that sender? Click to size up