Just a small question on Magic Pen.

Magic penetration affects all magic damage.

Magic pen simply increases the magic damage done, by reducing the enemies Magic resist. But Magic Damage isn't AP, But the comination of AP Scaling and base stats. (And what ever other source of magic damage you might have.)

A mage with Low Ratios but High base damage can still do fair amounts of damage without needing high levels of AP. And because Mpen items are typically cheaper than raw AP items, it's the go to option for mage supports who lack gold, but still want to do lots of damage.