Dunno if you've found the answer by now, since it's been over a year and not sure if this is what you're looking for, but this appears to be the root folder of iCloud Drive on the local computer; only the folder structure and downloaded items appear here in Terminal:
/Users/[username]/Library/Mobile Documents/com~apple~CloudDocs/
For some reason the Desktop folder is absent here, but Documents and everything else was there. Note that I could only access that folder in Terminal; Finder automatically jumped me over to the iCloud Drive pane when I entered it into "Go to Folder...". Notably, while Desktop and Documents are absent from the User Folder in Finder, they do appear there in Terminal. But the path above is the only place I found items in the root of my iCloud Drive.
Experimentally, I copied a file into a subfolder of that path in Terminal and was able to access the file on my iPhone, so it does seem to sync from there. I'm still on Catalina, if that makes a difference to anything.