You can find weapon mod Blueprints by playing Side Missions and Projects, and then construct them at the craft bench and some mods are even unlocked as Perks by spending SHD Tech at the Quartermaster. All of these Weapon Attachments can have Attachment mods which increase and decrease certain stats of your weapon. Weapons can have several different attachments: Scopes, Muzzles, Underbarrels and Magazines.