The Two-Weapon Fighting rule says:
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.
If either weapon has the thrown property, you can throw the weapon, instead of making a melee attack with it.
With Two-Weapon Fighting, I can only add my ability modifier to the damage of the first attack. Even if I get the Dual Wielder feat (PHB, p. 165), it doesn't say anything about adding the ability score to the damage.
The Crossbow Expert feat (PHB, p. 165) says:
Thanks to extensive practice with the crossbow, you gain the following benefits:
- You ignore the loading quality of crossbows with which you are proficient.
- Being within 5 feet of a hostile creature doesn’t impose disadvantage on your ranged attack rolls.
- When you use the Attack action and attack with a one-handed weapon, you can use a bonus action to attack with a loaded hand crossbow you are holding.
When I attack using the bonus action granted by Crossbow Expert, do I add my ability modifier to the damage? I don't know whether I should follow the Two-Weapon Fighting rules, or roll damage as for a normal weapon attack.