Plugins are installed by adding a subfolder to ASSETS/plugins -folder. If a folder name starts with an underscore (such as "_old_spxPlugin") it will be ingored at load time.
Each plugin folder must contain "init.js" file which will define the UI for the plugin and related functionality.
A plugin is often visualized in the SPX UI as a button and this button can then open a new user interface window. These additional user interfaces are called Extensions. Extensions can be used to create customized workflows or integrations that are difficult or cumbersome to handle with rundown-based templates.
See these extensions for more: