There are two ways a folder appears in the Finder sidebar…
- you drag the folder icon from the Finder widow and drop it on the sidebar or
- It is placed there in the Finder Preferences
In either case it is an alias for an actual folder on an attached drive, a server, or iCloud.
- By default the Documents Folder icon in the Finder sidebar is an alias for the Documents folder in your user folder.
- Open Finder > Preferences > Sidebar and if Documents under iCloud is checked then it is an alias for the iCloud Documents folder. This is activated, or deactivated in System Preferences > iCloud > iCloud Drive > Options > Desktop & Documents Folders. NOTE: the iCloud Documents folder is a mirror of the ~/Documents folder on your user account so for all intents and purposes they are one in the same.
- If you have dropped another Documents folder on the Finder Sidebar then it is an alias of that folder.
In the first two cases the Finder Sidebar icon is one in the same
as the Documents folder in your user folder. In the third situation
But you can always drag the icon out of the Finder Sidebar and then drag the Finder icon for the Documents folder in your user home folder onto the Sidebar to be absolutely sure which it is aliasing.