Unityのエディタ拡張の書き始めに迷う時のテンプレート https://docs.unity3d.com/ja/current/ScriptReference/EditorGUILayout.html

BarGenerator.cs

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

namespace Kanonji.Editor {

	public class BarGenerator : EditorWindow {

		private GameObject subject;

		[MenuItem("Tools/BarGenerator")]
		private static void Main() {
			EditorWindow.GetWindow<BarGenerator>("BarGenerator");
		}

		private void OnGUI() {
			this.subject = EditorGUILayout.ObjectField(this.subject, typeof(Object), true) as GameObject;
			if (GUILayout.Button("Do something")) {
				this.DoSomething();
			}
		}

		private void DoSomething() {
			Debug.Log(this.subject);
		}
	}
}

BazWindow.cs

using UnityEditor;
using UnityEngine.UIElements;

namespace Kanonji.CodeGeneration.Editor {

	public class BazWindow : EditorWindow {

		[MenuItem("Tools/Baz")]
		public static void OpenWindow() => GetWindow<BazWindow>("Baz Window");

		public void OnEnable() {
			var root = this.rootVisualElement;

			var box = new Box();
			box.Add(new Button() {
				text = "Example Button"
			});
			box.Add(new Label() {
				text = "Example Label"
			});
			root.Add(box);
		}
	}
}

FooGenerator.cs

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

namespace Kanonji.Editor {

	public class FooGenerator {

		[MenuItem("Tools/FooGenerator")]
		private static void Main() {
			//Do something.
		}
	}
}