I dont know if it works like macOS since I dont use macOS - but it's not a simple copy-to-cloud. It actively replaces file handling in those folders, which breaks a bunch of applications like games that save stuff in Documents/My Games..
Is the "Desktop & Documents Folders" sync option in iCloud on by default? I've never used that feature, and it's a bit buried so it's hard to enable accidentally, but I haven't set up a new Mac from scratch in a long time so I don't know if it is a trap for new users the way OneDrive is.
It isn’t until you explicitly log in to your appleID and choose to do it during setup. It is very clearly laid out. Onedrive bills itself as essential to windows’s functionality.
