"Get Connected Folder" flow action can be used to get a single attachment of type "folder" for a Salesforce record. If you have only attached one folder to a Salesforce record, it can be used a "Record's folder" in external storage where all files related the Salesforce record are stored.
1 - Add Get Connected Folder Action to your Flow
In your flow builder, search for the element named "Get Connected Folder". You can also find this action in the CloudFiles category when you click "action" element in "Add Element" box. Select the action to insert it into the flow.
2 - Configure Parameters
This flow action takes only 1 parameter, which is "Parent Id". This is the Id of the Salesforce record for which you want to retrieve the attachment. This can come from flow's "Record" variable in record triggered flows.