The Addons button appears in the launcher's bottom bar once World of Warcraft is installed in the Battle.net prefix. It covers every version of the game it finds there: Retail, Classic, Classic Era, Anniversary and the test realms.
WowUp, the full addon manager
Install WowUp downloads WowUp, about 120 MB, into the launcher's data folder and opens it as its own window with your WoW installs already set up. WowUp searches, installs and updates addons from CurseForge, Wago and WoWInterface. Remove WowUp deletes it again.
Manual addons
Pick the game version, then:
- Open AddOns folder opens that version's AddOns folder in your file manager.
- Install from ZIP… installs an addon archive you downloaded.
- Install from link takes the address of a .zip file or of a GitHub repository. For a repository, the launcher picks the release file that matches the game version.
- Installed lists the addons in that version, each with a Remove button.
The launcher does not update manually installed addons. Use WowUp for that.