To listen to tags when they are selected from a component dialog - you want to use CQ.tagging.TagInputField.
You can code xtype listeners using the APIs documented here:
https://docs.adobe.com/docs/en/cq/5-6-1/widgets-api/index.html
For example - search for the class: Class CQ.tagging.TagInputField
Each data type has its own event handlers:
Then you code in a custom xtype (a JS file) where you can code against a given event handler.
See this article as an example:
https://helpx.adobe.com/experience-manager/using/dynamically-updating-aem-custom-xtype.html