In any email client, reply returns the message to the sender. How it does that depends on fields in the message header. If there is a Reply-to: field, the message will be sent there. If there is no Reply-to: field the message will be sent to the From: field. These two addresses are configured by the sender in their account settings, or added automatically by their server. They have nothing to do with CC fields.
In any email client, reply to all returns the message to the sender as I have just described, and also to any other recipients specified in the To: and CC: fields.
