> I believe the main feature they need is to sync the full file tree without having to download the actual files until needed. I don’t know how others do it, but I assume that you’d have to manually maintain mappings between a subtree in the cloud to one on disk, or alternatively run out of disk space if you want to have everything synced

I thought the whole point of file syncing software was to sync all files. If I do not have internet access, then would I not be guaranteed to use all the files in my Dropbox folder?

I only use iCloud Drive, does it do the same?