Can I store files in iCloud but not on my devices until needed?

"Optimize Storage" will automatically manage that for you.

On a Mac, go to Apple menu > System Settings > [username] > iCloud > Drive > Optimize Mac Storage.

Also see the following screenshot for that setting:

There is a similar option for iCloud Photos.

JRT9 wrote: … I would like to save memory on my devices by having files that are seldom used residing in iCloud but not on my devices until needed.

I don't use Optimize, but I do have a file or two that I keep at iCloud rather than on my Mac. Even without "Optimize" turned on, you can choose to remove a file from your local iCloud Drive folder. A right click on a file will produce a sub-menu

where you can choose "Remove Download" and replace the actual file with a link to iCloud.com. Then, when you double click on that link version of the file, it will download the file and open it in the usual app. You will again be dealing with a local file.

See this for some background:

Understanding iCloud Drive - Apple Community