Addon User Guide
For core addons (those downloadable directly from Jsoftware) and the base library, there is a Jqt GUI interface called Package Manager and a console interface using the jpkg verb.
Other addons can be installed from github using the install verb.
In each case you need internet access as well as write access to the ~addons folder. No personal data is sent to Jsoftware or github.
In Jqt, select menu Tools|Package Manager. This lists the installed and available packages for selection and installation.