Can no longer make sense of OneDrive after new installation of Windows

I've been using OneDrive for years to backup the contents of my documents folder. Nothing fancy, just a means of ensuring that everything I create is safely stored in the cloud.

It worked perfectly for me until the other day, when I built a new PC with a fresh installation of Windows 10. Now, OneDrive has created a new version of my documents folder (which is on an external hard disk) in the 'Users' folder on my C drive. Furthermore, it isn't uploading new files I create in my original documents folder.

Is there any way I can regain control of OneDrive so that I can make it behave as before. Actually, just understanding what the hell it's doing now would be a start.