I would recommend PSG Art Tutorial as a good starting place, and actually paint your own copies of each exercise: http://androidarts.com/art_tut.htm#the_onion More in a similar vein: http://wiki.polycount.com/wiki/Concept_Fundamentals
To understand why metals are lit the way they are, you should understand commercial photography. Photographers spend a lot of time and effort on crafting precise reflections on metallic and mirrored surfaces. For example: https://theslantedlens.com/2023/how-to-photograph-shiny-metal-lighting-tutorial/