Question: Would a Dwarf's +2 to constitution be added to their ability stat, or just their modifier when making a roll?

Answer: 
**It's added to the ability stat.**

If you are using point buy then any racial bonuses are added at the end. If you are rolling then you add the bonus after assigning each roll to a particular ability.

This would typically provide a +1 bonus to CON based rolls.
Ability modifiers in general, and racial ones in particular, affect your ability score, not just rolls that depend on it. So for stats like Con and Str, where individual ranks matter for derived stats, you would benefit from an increase to those derived stats in addition to the bonus on rolls.