First, your agency needs Google Drive connected — that's done by the admin under Settings → Integrations → Google Drive. There the agency chooses a root folder (e.g. "Clients") inside the agency's Drive.
On the client profile → tab Client info → section Google Drive, Senly automatically matches an existing subfolder with the same name as the client. Match correct? Then you immediately see all files in that folder within the portal.
No existing folder found? Click Create client folder to automatically create a new subfolder under the root.