Since I cant find it clearly stated or I cant read; when making an attack does the extra attack add in the Ability modifiers?

IE LvL 5 Fighter with 16 strength and a war hammer do first attack 1d8+3 extra attack 1d8+3

Well....it is EXTRA attack.  So, instead of attacking once, it's twice.  You treat them both equally.  So, yes, you add the modifiers to both.  ALSO, being extra attack, if you critical fail the first attack, you still get the second attack.  I've seen DM's rule against this, but that is multi attack rules, and is different from extra attack.  There, I said it, but that's how I DM and every DM can rule at their discretion.

Edit: Also, not a dumb question at all.  It is a topic that is commonly debated amongst established tables.  Lastly, there are no dumb questions, particularly when it comes to trying to improve your own gameplay. ;)

Rules will say you don't get to add your ability modifier if that is the case. The best example is Two-Weapon Fighting's bonus action attack.

Two-Weapon Fighting
When you take the Attack action and attack with a light melee weapon that you're holding in one hand, you can use a bonus action to attack with a different light melee weapon that you're holding in the other hand. You don't add your ability modifier to the damage of the bonus attack, unless that modifier is negative.

Therefore - because Extra Attack doesn't have that clause - you most certainly can add the ability modifier.

Basically the general rule is that you add your ability modifier to the damage of a weapon attack. The specific rule (if it exists) is that you don't. Without the specific rule - you use the general rule.

All weapon attacks add an ability modifier unless they specifically say they don't (Two weapon fighting specifically says it doesn't).