what if user creates a file / folder in Sharepoint Library Root via Windows File Explorer?

Background information: I’m new to Office 365 / Sharepoint online, trying to roll out the usage for our comapany. I like to move most important stuff from file server network share to Sharepoint Online. I already managed to create Team- / Communication-Sites so I can build a root structure for the files, and I found out how to sync them to the local machines. By activating sync for one of the sites Windows creates a new Library with an Enterprise Building as a logo, serving as root directory for the document folders of the synced sites.

And here comes my question: What if users (used to file server network share!) create a new file or folder in that Library root via Windows File Explorer? It will not sync back to Shatepoint because there is no site defined (right?). But Windows allowed me to do so: Any suggestions on how to handle this / how to prevent users from creating non-synced files inside their local Sharepoint Library?