Prevent Mac and iPhone from syncing recent folders

If you don't want a file or folder to sync, then just remove it from the iCloud Drive folder. The purpose of iCloud is to make exactly the same iCloud Drive files available on all connected devices.

For a file that's already downloaded to your phone, you can select the file and use the More (•••) button at the bottom right to choose "Remove Download:"

It's replaced by a pointer that will download the file from iCloud when you tap it.

If you want files to automatically be available but not taking up space until you need them, then you can use Optimize Storage with iCloud Drive.