iCloud Drive stores files in iCloud and keeps them in sync across your Mac, iPhone, iPad and iCloud.com. With Desktop & Documents sync turned on, those two folders follow you to every device too.
1Turn on iCloud Drive
Open System Settings, click your name at the top of the sidebar, then iCloud, then iCloud Drive (or Drive), and turn it on. iCloud Drive appears in the Finder sidebar.
2Sync Desktop and Documents
In the same iCloud Drive settings, turn on Desktop & Documents Folders. Your existing files upload to iCloud and the folders move under iCloud Drive in the Finder sidebar. If another Mac already syncs these folders, the second Mac's files go into a folder named after it.
Make sure you have enough iCloud storage before turning this on. A large Documents folder can fill the free 5 GB plan quickly.
3Optimise Mac Storage
With Optimise Mac Storage on, macOS keeps recent files on the Mac and leaves older ones in iCloud, downloading them when you open them. A cloud icon marks files that aren't downloaded. Control-click a file or folder and choose Download Now or Keep Downloaded to keep it on the Mac, or Remove Download to free space.
4Check sync status
The Finder shows sync status next to files and at the bottom of the sidebar. Use View > Show Status Bar for more detail. You can also see files at iCloud.com.
5Turning it off safely
If you turn off Desktop & Documents Folders, your files stay in iCloud Drive, and your local Desktop and Documents folders start empty. Copy back anything you need first. If you turn off iCloud Drive entirely, macOS offers to keep a copy of the files on the Mac; say yes.