When I view the Lists page in Contacts, among other lists that I created, I see the following three lists: All Gmail, All iCloud and All iPhone. I don’t know how these lists were created and what they are. What are their significances?
From what I can see, they are all of the contacts that are available as defined in that particular service. For example, “All Gmail” contains all of the contacts that you would have created on Gmail. The list is created automatically by Contacts.
The list “All Contacts” is all contacts across all services.
A list may be empty if you have not defined any contacts in that service. For example, I keep all of my contacts in macOS/iCloud but have not defined any contacts in Exchange/Outlook.