React to something entering a trigger

private void OnTriggerEnter(Collider other){
    Debug.Log("Fish entered, name is:");
    Debug.Log(other.gameObject.name);
}

GetComponent on itself

void Start(){
    GetComponent<CatScript>().healthPoints = 5;
    GetComponent<CatScript>().Meow();
}

GetComponent on other GameObject

public GameObject dog;

void Start(){
    dog.GetComponent<DogScript>().healthPoints = 5;
    dog.GetComponent<DogScript>().Bark();
}

Making variables/functions accessible in DogScript

public int healthPoints = 1;

public void Bark(){
    Debug.Log("Dog is barking!");
}

Switching Scenes

void Start(){
    UnityEngine.SceneManagement.SceneManager.LoadScene("SceneName");
}

Called when mouse pointer enters the object's collider (requires Collider component) - using Old Input System

public Material highlightedMaterial;
    
    void OnMouseEnter()
    {
        GetComponent<Renderer>().material = highlightMaterial;
    }

Called when mouse pointer exits the object's collider

public Material normalMaterial;
    void OnMouseExit()
    {
        GetComponent<Renderer>().material = normalMaterial;
    }

Called when mouse button is pressed while over the object

void OnMouseDown()
    {
        Debug.Log("Clicked: " + gameObject.name);
    }

Called every frame while mouse is over the object

void OnMouseOver()
    {
        transform.Rotate(Vector3.up * 50f * Time.deltaTime);
    }

Detect when object enters trigger zone

void OnTriggerEnter(Collider other)
    {
        Debug.Log("Object entered: " + other.name);
    }

Check for specific name

void OnTriggerEnter(Collider other)
    {
        if (collision.gameObject.name.Contains("Player"))
        {
            Debug.Log("Player entered!");
        }
    }

Detect when object collides

void OnCollisionEnter(Collision collision)
    {
        Debug.Log("Collided with: " + collision.gameObject.name);
    }

Detect key press

void Update()
    {
        if (Input.GetKeyDown(KeyCode.Space))
        {
            Debug.Log("Space pressed!");
        }
    }

Basic movement with axes. Horizontal and Vertical are pre-named by Unity but you can make your own.

public float speed = 5f;
    
    void Update()
    {
        float h = Input.GetAxis("Horizontal");
        float v = Input.GetAxis("Vertical");
        transform.Translate(new Vector3(h, 0, v) * speed * Time.deltaTime);
    }

Jump with button

void Update()
    {
        if (Input.GetButtonDown("Jump"))
        {
            Debug.Log("Jump button pressed!");
        }
    }

Detect mouse clicks

void Update()
    {
        if (Input.GetMouseButtonDown(0))  // Left click
        {
            Debug.Log("Left clicked!");
        }
    }

Mouse movement for camera

public float sensitivity = 2f;
    
    void Update()
    {
        float mouseX = Input.GetAxis("Mouse X");
        float mouseY = Input.GetAxis("Mouse Y");
        transform.Rotate(-mouseY * sensitivity, mouseX * sensitivity, 0);
    }

Basic debug logging

Debug.Log("Hello from Unity!");
Debug.Log("Player health: " + 10);
Debug.Log("The time since start: " + Time.time);

Warnings logging

Debug.LogWarning("Warning: Object not found!");

Error logging

Debug.LogError("Critical error: Object not found!");

Spin windmill continuously

public float rotationSpeed = 50f;
    
    void Update()
    {
        transform.Rotate(Vector3.forward * rotationSpeed * Time.deltaTime);
    }

Spin with axis as variable

public Vector3 rotationAxis = Vector3.up;
    public float speed = 100f;
    
    void Update()
    {
        transform.Rotate(rotationAxis * speed * Time.deltaTime);
    }