We’re in the process of updating the documentation for Zotero 5.0. Some documentation may be outdated in the meantime. Thanks for your understanding.
Plugins can be divided in two groups: XUL plugins, which closely integrate with the Zotero client and external plugins, which are more likely to rely on the Web API to access Zotero libraries. XUL plugins are built on the same technologies as legacy (pre-WebExtension) Firefox extensions.