I know upgrading a hero increases their atk, hp and def.I also know enhancing a hero increases their..

Troop skills are either passive or active. Most "Economy" hero skills are passive which means just by owning the hero and achieving the unlock level (e.g., Upgrade levels 31, 71, 131, etc.), you get the benefit.

But several heroes have specific Fighter, Shooter, Rider or Troop skills and must be added to an attack (or defense) on another sanctuary, zombie lair, or zombie, to name a few. You will get the unlocked benefit for the troop.

Here's an easy example. You are shooter focused and send 1000 shooters to kill a low level zombie. If you have Malcolm upgraded and evolved to level 71, you'll get a 5% HP bonus for those 1000 shooters, a 5% DEF bonus and a 5% ATK bonus.