Based on your questions related to your Firestore database, below are my answers:
Since you still have the document intact in the Production environment, you can export the document from Production and then re-create it in UAT. You can use the Cloud Firestore managed export and import service to recover data if it’s accidentally deleted or to export data for offline use. You have the option to export all documents or just specific collections. Similarly, when importing, you can bring in all the data from an export or choose to import only certain collections.
How the document works in your app depends on how your app accesses it. If you recreate the document in UAT with the same ID as in Production, it should work fine. If the document has an auto-generated ID and you create a new one in UAT, your app might not find it unless you update the app to handle the new ID.