How OneDrive backs up library folders like Documents

I just activated OneDrive on my Windows 10 1803 laptop. The main reason I am interested is the new feature that allows one to backup key library folders such as Documents. However, I am trying to determine if, in selecting that option, will OneDrive then move the Documents folder under the OneDrive folder, or will it sync in place. I currently have my standard Documents and Pictures folders on a separate data disk D:. I set up OneDrive there, also, based on a note I saw that the folders must be on the same drive. However, I don't want to move my Documents directory as it has about 35,000 files in it, and I regularly back it up locally using Retrospect. If I move the folder, I will have to reconfigure Retrospect, and it will probably see 35,000 new files and back them all up again. Not to mention that I don't want to access and use those folders locally through OneDrive.

It would seem that the "feature" in OneDrive to backup those library files is not really a feature if it does move the folder. It is rather a user interface change that moves the folder for you. However, I am not confident as I cannot find any mention of this in the Microsoft help and documentation.

Thanks for any input.

Thanks, bro67. I found that link for how to set up the backup of the library folders. What I cannot find is whether they will stay where they are, e.g. /[username]/documents, or be moved under the OneDrive folder, e.g. /[username]/OneDrive/Documents. I don't want the folder to move for the reasons I mentioned. Obviously, if they are moved under the OneDrive folder they will be synced.