How to use iCloud as main storage location for all documents, application files, etc?
What you want to do is possible, assuming your internet connectivity is good on all devices (which I assume it is) and that they’re running a supported version of iOS and macOS.

More specifically, iCloud Drive requires iOS 8.0 or above and Mac OS X Yosemite or above.

If your Macs are running macOS Sierra or above, you’ve also got the option of Optimized Storage. I encourage you to read How to free up storage space on your Mac for detailed information.

I also encourage you to read Manage your photo and video storage to help you understand the best ways of doing this on iOS devices if you’re wanting everything stored online.

And, assuming you’re running iOS 11 or above on your iOS devices, you’ll want to use the Files app. Read Use the Files app on your iPhone, iPad, and iPod touch for more information.