Extension Methods

Vector3 and Transform belong to Unity, so you cannot add a method to them. An extension method is a static method that C# lets you call as if it were theirs. These two save lines that come up in every project:

public static class UnityExtensions
{
    public static Vector3 WithY(this Vector3 v, float y)
    {
        return new Vector3(v.x, y, v.z);
    }

    public static void DestroyChildren(this Transform parent)
    {
        foreach (Transform child in parent)
        {
            Object.Destroy(child.gameObject);
        }
    }
}

// in any script
transform.position = transform.position.WithY(0.5f);
inventoryPanel.DestroyChildren();

Three things make an extension method: a static class that is neither nested nor generic, a static method, and this before the first parameter. That parameter is the value on the left of the dot, so transform.position.WithY(0.5f) compiles to UnityExtensions.WithY(transform.position, 0.5f). Vector3 is a struct, so v is a copy: WithY cannot change the caller's vector and returns a new one instead. That is also how it gets round transform.position.y = 0.5f;, which does not compile.

The static class is not a MonoBehaviour. It lives in a .cs file of its own, any name will do, and never goes on an object. Methods a MonoBehaviour inherits, such as Destroy, need their class here: Object.Destroy. Destroying children inside the foreach is safe, because Unity removes them only at the end of the frame.