9.13The canonical picture

Memorize this:

           MOBILE
              │
              ↓
      Mobile Experience API
              │
              ↓
       Customer Process API
          ┌────┼────┐
          ↓    ↓    ↓
     Salesforce Billing Loyalty
      System API APIs   API
          ↓
     Salesforce

Now you can explain the whole model.