Final answer:
The add-ons and extensions created by other users and made available to end users on a platform are called third-party plugins or user-generated plugins.
Step-by-step explanation:
The add-ons and extensions that were created by other users and made available to end users on a platform are commonly referred to as third-party plugins or user-generated plugins.
Third-party plugins enhance the functionality of a platform by providing additional features or customizations that are not included in the default installation. They are typically developed by independent developers or companies who leverage the platform's architecture and APIs to build these extensions.
For example, in the WordPress content management system, there is a vast library of third-party plugins that allow users to add features such as contact forms, social media integration, or search engine optimization tools to their websites. These plugins are developed by a wide range of developers and are made available in the WordPress plugin directory for users to install and use.