The easiest way is to create a pointer / reference in your class which points to the object you’re going to interact with. During level design, when you place the object into the level, you select the object it interacts with. So, if you place a light switch in the level, it has a reference to a light bulb. You place the light bulb into the level and a light switch, and then you tell the light switch about the light bulb by giving it a reference to the specific light bulb you want to toggle on and off when the switch is used.
If you realize that switches can toggle many things, not just light bulbs, then you could create a more abstract implementation using interfaces and just pass in a reference to an actor. If the referred to actor implements the interface, it will do its thing via an interface call. So, a switch attached to a light bulb toggles the bulb on and off, a switch wired to a blender will make the blender blend. It’s up to the individual objects themselves to decide how they want to react when they are switched on and off (ie, how they want to implement their interface call).