Scene Hierarchy View

You can customize the scene hierarchy view in the same way as the project view by registering a callback to UnityEditor.EditorApplication.hierarchyWindowItemOnGUI.

using System.Collections.Generic;
using UnityEngine;
using UnityEditor;

[InitializeOnLoad]
private class MyProjectViewExtentions
{
    private static List<int> favoriteInstances = new List<int>();
    
    private static MyProjectViewExtentions()
    {
        EditorApplication.hierarchyWindowItemOnGUI += DrawHierarchyItem;
    }

    private static void DrawHierarchyItem(int instanceID, Rect selectionRect)
   {
        Rect frame = new Rect(selectionRect);
        {
            frame.x += frame.width - 20f;
            frame.width = 18f;
        }

        if (favoriteInstances.Contains(instanceID))
        {
            if (GUI.Button(frame, "♥︎", GUI.skin.label))
            {
                favoriteInstances.Remove.Remove(instanceID);
            }
        }
        else if (selectionRect.Contains(Event.current.mousePosition))
        {
            if (GUI.Button(frame, "♡️", GUI.skin.label))
            {
                favoriteInstances.Add(instanceID);
            }
        }
    }
}