agrandj wrote:
All files synced with iCloud will be stored on the server under my iCloud ID, yes.
The question is whether you can elect to sync only specific files/folders (i.e. continuously synchronize) with a physical hard drive. Google Drive allows you to choose which items are downloaded and synchronized with your device. Everything else is stored remotely (or on G Drive/iCloud) and remains accessible as long as you can connect to the internet. Here is a screenshot of what I am describing:

Just to be as clear as possible, another example: say that I have a computer with a 1GB hard drive, and I have an iCloud account with 200GB of storage, which I have filled to capacity. Since my computer can only store 1GB on its own, I cannot sync all of my iCloud folders. So, can I choose to sync only 1 folder that contains 1GB? In other words, can i choose to only download and synchronize 1GB of data instead of all 200GB?
OK, so you cannot "selectively" sync iCloud Drive files on a Mac. You can selectively download to a device - the iCloud Drive app on a device will show all of your available files, and you can download whichever ones you want. But on a Mac, iCloud Drive downloads your most recent file as space allows, leaving the remaining ones on iCloud Drive until you manually download them. You can also turn on Optimization which will free up even more space on your Mac, and put more items on the Drive:
The files that you keep in iCloud Drive use your iCloud storage. And as long as you have enough space in iCloud and on your device, you can store as many files as you like.
If you need more space on your device, iCloud Drive can help. Your Mac keeps all of your recent files on your computer, but keeps your older ones only in iCloud, ready for you to download when you need them again. If you want to free up more space on your Mac, we can help. On your iPhone, iPad, or iPod touch, you can search and browse files from all of your devices and download them only as you need them.

Here are a couple of iCloud Drive support articles that might help:
iCloud Drive FAQ - Apple Support
iCloud Drive Usage and Instructions
Cheers,
GB