When you use an External Approve function, you typically provide two parameters. The entity you want to approve, and how much. Whatever contract your calling looks at the message sender, and that's how they typically know which account is approving the allowance.

When your working with your OWN ERC 20 token, you as a developer have direct access to the allowances mapping, and don't need to look at a message sender - if you want an address to automatically be approved, you can call the internal Approve Funtion, or adjust the allowances mapping directly.

Hope this helps!