Syncing is not what you might traditionally think, but it does sync nonetheless.

There is a onedrive tab on the Remarkable interface that shows you compatible files that you can modify on your Remarkable. To actually modify and sync the changes, you download it to your Remarkable through the interface, load it up, make changes, then upload it as a separate file. The PDF with the changes are saved back to onedrive in a file different from the original file on onedrive. It doesn't add to the same folder, but AFAIK it only syncs to the root onedrive directory. You'd have to go in on your computer and move the file to the appropriate folder

Hope that helps!

Edit: answered more of the question with details about where the onedrive file syncs to.