To upload files to OneDrive automatically, just copy or move the files to your OneDrive folder using the Finder, or save them in your OneDrive folder from an app. If you add, change, or delete a file or folder on the OneDrive website, the file or folder is added, changed, or deleted in your OneDrive folder and vice versa. This folder is kept in sync with OneDrive.
When you install the Microsoft OneDrive sync app for Mac, a copy of your OneDrive is downloaded to your Mac and put in the OneDrive folder.