https://docs.joomla.org/Application_execution_order
The plugins are imported via JPluginHelper::importPlugin('system')
The plugin can be triggered by events :
https://docs.joomla.org/Plugin/Events/System
https://docs.joomla.org/Plugin/Events
https://docs.joomla.org/J4.x:Triggering_content_plugins_in_your_extension