To give you an example to the above:
in form-login.php, in the root folder of the Vantage theme, you will find this line:
<?php require APP_FRAMEWORK_DIR . '/templates/form-login.php'; ?>
This tells us that we need to look in "APP_FRAMEWORK_DIR", which is the folder /framework/, for a sub-folder /templates/ and therein for a file named form-login.php
So, if you need to apply changes to the login form, you do it in /wp
-content/themes/vantage/framework/templates/form-login.php . Bear in mind that if you make functional changes here (such as additional information from the user), you will need to handle also the writing of the information you collect into the database.