I'm not exactly getting what you need that's not provided ootb via either search fallout topic - they both link to the catalog item, there shouldn't need to be any customization needed.
If you want to dive deeper into how AI Search result displays are configured, look up EVAM: Entity View Action Mapper (servicenow.com) - though warning EVAM is pretty complex and I don't mess with it.
If you're not getting what you want from the ootb search fallbacks, then create your own custom fallback search topic and perhaps use the ootb Search Catalog Item topic block. The user utterance should be fed into the query_term input of the topic block, then the topic block will display the results and let the user choose the result. Then that result will be fed to the catalog_item_id output, which you can then use to dynamically populate a Link Output pointing the user wherever you'd like (i.e. to a specific Portal, etc.). If you don't like the way that works, then you'll have to do something more custom, such as using the ootb AI Search/Run AI Search or Contextual Search topic blocks which'll give you even finer control with how the results are output.
