You could use ArcGIS Xray. Just make sure to run it against the Asset Package in a File GDB. Don't think it works in Pro and ArcCatalog can't see the UN. This will dump the schema out into a set of excel files as well as a .htm with the entire schema that is easily viewable through hyperlinks within the page.
Thank you for posting your question. Just wanted to let you know that a data dictionary widget is currently in development for ArcGIS Experience builder. The targetted release date is not yet determined.
For an example of what this widget could look like once configured against a service, please take a look at this application: ArcGIS Solutions Utilities Data Dictionary - this is the data dictionary for the Electric Utility Network data model provided with the Electric Utility Network Foundation solution- What is the electric data model—Design Pattern | ArcGIS Solutions for Electric