ICloud
Internal
Overview
iCloud is a syncing system, where the data is centrally stored in the cloud, accessed by multiple devices and locally cached by those devices. iCloud drive is setup so that anything stored locally syncs to the cloud and anything in the cloud syncs locally. Deleting from one location would delete from another.
Configuration
System Preferences → Internet Accounts → <User>'s iCloud
iCloud Drive
iCloud Drive Configuration
System Preferences → Internet Accounts → <User>'s iCloud → iCloud Drive → Options
Storage Capacity
Storage capacity used so far: https://support.apple.com/en-us/HT204247
Apple Menu -> System Preferences -> iCloud -> colored bar at the bottom of the window -> Manage.
Access it from command line:
/Users/ovidiu/Library/Mobile Documents/com~apple~CloudDocs
Local iCloud Drive File Storage
iCloud Drive has hidden shadow copies of the files in the local user library in ~/Library/Mobile Documents
iCloud Drive Cleanup
Log out from the iCloud account: System Preferences → Internet Accounts → <user>'s iCloud → Remove the account with the "-" option.