The login form’s block:
The login form’s template with the «Sign In» button highlighted:
The login form’s action
value (the URL where the form’s data will be sent):
https://github.com/magento/magento2/blob/8fd3e8/app/code/Magento/Customer/view/frontend/templates/form/login.phtml#L25-L25
https://github.com/magento/magento2/blob/8fd3e8/app/code/Magento/Customer/Model/Url.php#L117-L131
The server handles the data sent with the controller: