How can I modify the GUI of a default Unity window?

Question

How can I modify the GUI of a default Unity window?

E.g. I am trying to modify the Animation window. Here is what I am trying to do for the beginning:

using UnityEditor;
using UnityEngine;

[CustomEditor(typeof(Animation))]
public class AnimationExtended : Editor
{
    public override void OnInspectorGUI()
    {
        
    }
}

I would expect the code above to hide the content of the Animation window completely, but it does not. Why?

Here is what I mean by the Animation window.

Question which made me want to add a custom feature to the Animation tab.