To make it appear built-in you either

• decrease the depth of the fridge
• recess it further into the wall
• Build out the adjacent wall.

I see what youre saying. what i meant was the line down the wall. left of light switch. wood putty? plaster mud? caulk? how do i fill that gap

Assuming there is a tight bond and no different movement I would sand a bit to rough it up and use wood fill. Or you could just use caulk, but you will always see the seem.

Add a trim molding along that line and the edge of the wall….. guessing here