The Blender Extensions platform is the online directory of free and Open Source extensions for Blender.
The goal of this platform is to make it easy for Blender users to find and share their add-ons and themes, entirely within the Free and Open Source spirit. The platform only offers GNU GPL compliant software.
In Blender 4.2 LTS or newer, open Preferences › Get Extensions.
The first time you will be greeted with a message asking you to "Allow Online Access".
Once online acess is granted, you will be able to see the extensions listed inside Blender.
You can also explore the extensions in this website and install them via drag & drop into Blender.
Note: Extensions don't update automatically. You need to manually check for updates from the dropdown menu in top-right, then "Check for Updates".