The CAST Extend website is the main "hub" for managing extensions. Extensions can be downloaded (official and custom) and uploaded (custom) in a modern graphical interface accessible in your browser. Access requires registration but is open to all:
CAST Extension Downloader
The CAST Extension Downloader is a software application bundled with CAST AIP (from AIP 8.x). As its name suggests, it provides a convenient method for downloading extensions. It is only available with CAST AIP and cannot be used to upload extensions. You can find out more about this here.
Nuget.exe Command Line Interface (CLI)