According to the Azure AD B2C FAQ:
Can I use Azure AD Connect to migrate consumer identities that are stored on my on-premises Active Directory to Azure AD B2C?
Azure AD Connect is not designed to work with Azure AD B2C...
Then why is it displayed here? And what can you do with Azure AD Connect and B2C then?
The displaying of that link implies there's a relationship between the two of them (to me at least).
The FAQ is correct in stating that Azure AD Connect is not supported with Azure AD B2C along with several other features of regular Azure AD.
These features show up in the Users and Groups blade because that blade was built primarily for regular Azure AD. There is work underway so that this blade understands it's running in the Azure AD B2C context and only shows applicable features.
Then why is it displayed here?
This is because that when you want to manager users and groups in Azure AD B2C, you must use Azure AD to manage it. Azure AD B2C cannot leave Azure AD. When you are using Azure AD B2C, you would have used Azure AD to authenticate Identity. As @Saca said, that blade was for Azure AD.
And what can you do with Azure ADConnect and B2C then?
That FAQ is right, but you can still use Azure Connect to sync on-premise users to Azure AD. You can also use the synced users accounts to login Azure AD B2C. But after syncing , the user name would changed to .onmicrosoft.com.
If you still want use your local account email address for the synced username, you can refer to this document and this official support article.
