sharepoint online – Moving Folders and Files From a Team Site to Sub-Site

You can use the SharePoint interface to move files and folder in same tenant.
Opening your Library (Use new experience), you have a MOVE option selecting a folder. After select the folder and click on MOVE TO, select the destination on right side.

IF this dont work for you, try to use the ShareGate or SharePoint Migration Tool (or try).

Other option is Sync the libraries using OneDrive. Map the two libraries and move the files manual, waiting the files sync.

All this options you dont need to customize anything. If you cant solve your problem, try PowerShell PnP.