Example of how to access another GameObject's component

GetComponentExample.cs

using UnityEngine;

public class GetComponentExample : MonoBehaviour
{
  // First we need a reference to the other gameobject
  // You can make a public field and assign a prefab in the inspector
  public GameObject somePrefab;
  
  // Or you can search for a gameobject and assign it to a variable
  private GameObject gameObjectFoundBySearch;
  
  void Start()
  {
    // You can find a gameobject by name
    gameObjectFoundBySearch = GameObject.Find("Sphere");
    
    // Or you can find it using the gameobjects tag
    gameObjectFoundBySearch = GameObject.FindGameObjectWithTag("Tag");
    
    // Tip: Don't use GameObject.Find or GameObject.FindGameObjectWithTag in Update since
    // it's a heavy operation that scales with the amount of gameobects you have in the scene.
    // Call it in start and save the reference in a member variable.
  }
  
  void Update()
  {
    // Now to acces a component on the gameobject,
    // simply use the generic GetComponent method
    TypeOfComponentGoesHere component = gameObjectFoundBySearch.GetComponent<TypeOfComponentGoesHere>();
  
    // Tip: You can also use GetComponent in Start() and save a reference to the component
    // in a member variable, so GetComponent isn't called every frame. This is called caching
    // the component and is a very good practice!
  
    // You can now use the public methods etc. on the component of the gameobject
    component.DoStuff();
  }
}