How do I transfer files from an extended sd card (Moto g4 plus) to a new sd card?

Extended memory is encrypted so it appears to the phone as just more internal memory. The best way would be to upload everything you want to save to Google drive (or another cloud storage option) then you can download it to your computer, or other device and still have a backup copy in the cloud.

 

Yes, as Davidi said, you may take advantage of some free cloud storage as a bridge to transfer your files.
Google Drive, Dropbox, One Drive, or Amazon Drive.

 

You can try an app like Motorola Migrate or Samsung Smart Switch.

That's why I have always resisted integrating my microSD card with internal storage: internal storage is not safe, in my opinion. If I have all my data in internal storage (which would include extended storage) and drop the phone, bad update, can't boot up, etc., how do I get to my data? I don't. It's as good as gone. That's why I keep my files on the card. If the phone breaks, I pop out the card, put it in another phone and keep going.