What does it happens when we charge our iPhone overnight? Does it Harms the battery??

No the phone will stop charging when the battery reaches 100%, you cannot overcharge it.

Apple recommends you charge it overnight.

Plugging your phone into the power adapter and letting it charge overnight....every night.....is the single best thing that you can do to preserve the life and performance of the phone's battery.

It is impossible for the phone to overcharge, so no worries about that.