Hello Jeyner Gil !

Let's review your marked questions:

1) https://github.com/unaio/una-vendor-test/wiki

2) https://github.com/unaio/una/wiki/Dev-Pages and https://github.com/unaio/una/wiki/Dev-Forms

3) https://github.com/unaio/una/wiki/Architecture

4) Forms manual from p.2 and https://github.com/unaio/una/wiki/Dev-API for exact examples

5) Just call a function sendMail in the necessary moments (defined in inc/utils.inc.php file):

 * @param $sRecipientEmail - Email where email should be send

 * @param $sMailSubject - subject of the message

 * @param $sMailBody - Body of the message

 * @param $iRecipientID - ID of recipient profile

 * @param $aPlus - Array of additional information

 * @param $iEmailType - email message type: BX_EMAIL_SYSTEM, BX_EMAIL_NOTIFY or BX_EMAIL_MASS

 * @param $sEmailFlag - use 'html' for HTML email message

 * @param $isDisableAlert - disable alert

 * @param $aCustomHeaders - custom email headers

 * @param $bAddToQueue - add message to email queue

 * @return true if message was send or false otherwise

How looks "the necessary moment" you may check in inc/classes/BxDolAccount.php file by the code and call of sendConfirmationEmail method.

6) In addon to p.4 you may use our auto-generated docs https://ci.una.io/docs/