Published Data Sources From Admin Insights From Tableau Cloud Cannot Be Joined By Tableau Desktop

You can join Admin Insights data sources with Tableau Prep.
Using Tableau Prep on the Web or Tableau Prep Builder, connect to any Published Data Sources in the Admin Insights project you’d like to combine. You can then join the data sources (joining fields available here), remove any unnecessary fields or add calculations as desired. In the Prep Flow, include Outputs to either new Published Data Sources or directly to an external database (Prep Builder version 2020.3+ required). This will write the Output to that location. If you choose to publish to Tableau Cloud, make sure not to overwrite the existing Admin Insights Published Data Sources. If you do, your new data source will be overwritten the next time the provided Admin Insights data source is updated.

Publish the Prep Flow to Tableau Cloud, and using Data Management, set up a recurring schedule to append new data. (If you do not have Data Management enabled, you can refresh the Prep Flow manually or from the command line.) Whenever the Prep Flow runs, data will be written to your new Output(s) or external database.