what is the best way to migrate data from one sharepoint online library to another sharepoint online library within the same site collection. Any third party tool to recommend?
Option 1: Move to
If you're just moving one set of files, try the "Move to" option in the library user interface.
Option 2: OneDrive Sync
Another alternative would be to use the library sync option for both libraries and then use Windows File Explorer to copy/paste the files.
Option 3: PowerShell
A third option would be to use PowerShell if this is something you need to script. A sample script for SharePoint Online document libraries is at the bottom of the following web page:
Here is info on getting started with the PnP PowerShell module you will need to run the script:
Option 4: Paid Utility
If you have funds and want a utility with a user interface, ShareGate is an example of the paid utilities available. They have a free trial available to test with, but the full version runs over $4,00 per year.