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 used in Graphical User Interface (GUI) mode 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 an installation of CAST AIP and cannot be used to upload extensions. You can find out more about this here.
Nuget.exe Command Line Interface (CLI)
Nuget.exe is used to upload extensions in CLI mode, i.e. from a batch file for example. It is only available with an installation of CAST AIP and cannot be used to download extensions. You can find out more about this here.