Learn to Load Resources by Name in Runtime in Unity Resources Folder
Resources folder empty in Build and public variables NullReferenceException - Unity Answers
Unity Real-Time Development Platform | .NET
Textures – The Knights of Unity
How to Use Unity's Resources Folder – The Knights of Unity
If you make Unity Editor tools, try the new Editor Foundations Design System: This covers helpful guidance, best practices, and other resources to help users build a better Editor UI. : r/Unity3D