Custom Card Integration - Where to begin

Thanks for the guidance. I have created a mock private app, and a UI extension using one of the starter templates. This screenshot shows exactly what we need. Is it possible to have the extension just call an endpoint in our 3rd party system to retrieve the data in real time so no data sync/transfer is needed? So on navigating to a company record, the card calls our external endpoint passing the company id to fetch the data it needs… If possible, this is the best scenario, our data can remain outside HubSpot and be a single source of truth, always up to date/accurate.