Efficiently changing other object's variables via triggers?

Other is only the collider on the gameobject that was triggered in the trigger event, but NOT the gameobject that contains the trigger script. The other collider is the default incoming parameter of the GameObject function OnTriggerEnter. You can name it whatever you want like.

 void OnTriggerEnter(Collider colliderThatWasHit)
 {
     colliderThatWasHit.transform.SendMessageUpwards("function", 1);
 }

Think of these default functions as Override functions without requiring the override keyword. There are a lot of them and each one has its own incoming parameters, or none at all. Some examples of these functions are...

 void Update() // runs every frame
 {
 }
 void FixedUpdate() // runs every physics frame
 {
 }
 void LateUpdate() // runs after Update. Like a secondary update, in case other objects update was dependent on other objects. Most common reason for this function to be used is Camera movement. That way the camera is always at its final position.
 {
 }
 void OnEnable() // occurs when an object is Enabled including when it is first created or a scene is loaded.
 {
 }
 void Awake() // occurs once, the very first function to run on every script when it is first initialized on game loading/starting or instantiating.
 {
 }
 void Start() // similar to Awake but runs afterward.
 {
 }
 void OnMouseDown() // only applies to objects with colliders, occurs when a mouse button is clicked on the object.
 {
 }
 void OnCollisionEnter(Collision col) // occurs when this object collides with another, and gives Collision data about the event.
 {
 }