On Cloud, you can assign multiple contacts to an account. You can set one of them as a primary contact for the account. This way, the primary contact's details of an account are auto-populated when you create opportunities and service calls for the account.
By default, the first contact you assign to an account is the primary contact for the account.
To set a contact as the primary contact:
1/ Go to "Accounts" in the left navigation menu.
2/ Click the account, and this will open its details view.
3/ Click the "Contacts" tab and you will see the contacts for the account.
4/ Click the 3-dot menu of a contact and click "Set as primary."
The contact's details will be auto-populated when you create opportunities and service calls for the account.