With macOS Mojave, you can store your Desktop and Documents folder in iCloud Drive and access them on all your devices. That means you can start a document on your Desktop, then work on it later from your iPhone, iPad, or iPod touch and on iCloud.com (make sure your iOS devices are running iOS 10 or higher, preferably the latest version). Everything automatically stays up to date everywhere.
Turn on Desktop and Documents on every Mac that you want to use with iCloud Drive. Here’s how:
From your Mac, go to Apple menu > System Preferences > iCloud.
Make sure that iCloud Drive is turned on.
Next to iCloud Drive, click Options.
Choose Desktop & Documents Folders.
In Finder, your Desktop and Documents folders move out of Favorites and into the iCloud section of your sidebar. (Represented by the blue icon with the smiling face, the Finder is the home base for your Mac. You use it to organize and access almost everything on your Mac, including documents, images, movies, and any other files you have.) If you add a second Mac Desktop, you'll find those files in the Desktop folder in iCloud Drive. A folder is created with the same name as your second Mac.
When you add your Desktop and Documents to iCloud Drive, all of your files move to iCloud and any new files you create are automatically stored in iCloud too. Then you can find your documents, spreadsheets, and presentations on all of your devices.
If you want to store your files in iCloud Drive and another cloud storage service, you can keep copies of your files in both, but you can't keep folders from a third-party cloud service in iCloud Drive. You can keep your other cloud service folders in a different place on your Mac, like the home folder.
Your files upload to iCloud each time your device connects to the Internet. Depending on your Internet speed, the time it takes for you to see your files on your other devices might vary.
On your Mac, you can find the files on your Desktop and in your Documents folder in Finder under iCloud. You can also search for the titles with Spotlight.
You can organize and combine folders from multiple Macs when you turn on Desktop and Documents on those computers. After you turn on Desktop and Documents on a second Mac, you'll find its files in folders with the same name as your second Mac inside your original Desktop and Documents folders. You can combine them manually, but iCloud doesn’t automatically merge the files in case you want to keep them separate. Any edits you make are automatically updated in iCloud Drive. There’s no need to manage your folders and files again on your other devices.
With the iCloud Drive app, it's easy to access and edit your files directly from your iPhone, iPad, and iPod touch. If you don't have the iCloud Drive app on your device, go to the App Store and download it. When you find the file you need in the iCloud Drive app, you can use it in any compatible app to edit it.
iCloud.com lets you access your Desktop and Documents folder from another computer browser. All of your files are easy to find, organize, and download directly from iCloud Drive. You can even drag files into the Desktop and Documents folders, and then find them later on your Mac.
Sign in to iCloud.com with your Apple ID. Go to iCloud Drive. Double-click the Desktop or Documents folder.
If you want to use the file or make edits, just click and download it to your computer. When you're done making edits, upload the file to iCloud Drive to see the latest version everywhere.
(This how-to is based on my experiences and info on Apple's support pages — where the images sometimes come from.)