laptop's processor fan

[prabin571]

does a laptop's processor fan (one on my left side) only work when it's

temperature increase or starts to run on startup?

 

[Barty1884]

It depends on the specific laptop, but generally the fans will run at a lower RPM while temps remain relatively low.
Usually you'll hear it ramp up on startup, then it'll settle down until it's required.

Some laptops are strictly passively cooled, others have numerous heatpipes & fans in place....... it's not really possible to give a definitive answer.