Weapon attachment customization was one of the most requested features and it's finally coming.
Every attachment is now a seperate item that you can interact with in your inventory. You will be able to collect attachment parts like scopes, silencers and red dot sights. When you hover over an attachment part, it will highlight the weapons it's compatible with.
You can then click to pick it up and then click on the weapon you want to attach it to. Now simply equip the weapon and it will have your attachment on it. If you want to remove the attachment from a weapon: Unequip it, and thensimply click on the attachment you want to remove.
NPCs are also able to use these attachments. For instance, if an NPC has a scoped weapon, they will be able to spot their enemies easier and they will have a better aim. You will be able to buy these weapon attachments from traders in the zone or via the online shopping. Your last option is hunting down other's for these attachments.