Does the radiator fan stay on constantly, or does it turn on and off?

Yes, the radiator cooling fan should turn on and off with the engine temperature, here is a guide on how to check the coolant level:

https://www.2carpros.com/articles/how-to-check-engine-and-battery-pack-antifreeze-levels

Coolant Type

Use Genuine Nissan Long Life Anti-freeze coolant or equivalent with the mixture ratio of 50% anti-freeze and 50% demineralized or distilled water when adding or replacing coolant.

Capacities


Cooling System Capacity (Approximate)

M/T Models With reservoir .................... 1 3/4 gal. (1 1/2 Imp gal, 6.8 L)

A/T Models With reservoir .................... 1 3/4 gal. (1 1/2 Imp gal, 6.7 L)

Please let us know what happens.

Well, even after adding the coolant, the engine temperature fluctuates between the middle and hot. I have shown a mechanic, he says there is a problem in the radiator fan, and it needs to be replaced. According to him, the fan turns on and off only when a/c is turned on, otherwise it does not work. Is it a wiring problem or does the fan need to be replaced?

Is there more than one fan motor? Here is a wiring diagram I found for the cooling fans and it shows two fan motors?