You can override login.phtml file using layout XML way or directly add template file to the theme level under Magento_Customer theme module.
login.phtml file used to display login form in Frontend area.
Theme Level Override,
app/design/frontend/<ThemePackage>/themename/Magento_Customer/templates/form/login.phtml
Module Level Override,
You can see a base definition for define login.phtml file at customer module, Magento_Customer/view/frontend/layout/customer_account_login.xml location.
Block name is customer_form_login which contains the login.phtml template.
you can see registration form by URL, <SITE_URL>/customer/account/login
You can override using Layout XML inside a module by <referenceBlock> node in your module customer_account_login.xml ,
Create a XML file, app/code/<Rbj/Modulename>/view/frontend/layout/customer_account_login.xml
In above path, <Rbj/Modulename> replace with your Package/Modulename.
<?xml version="1.0"?>
<!--
/**
* @author Rakesh Jesadiya
* @package Rbj_Modulename
*/
-->
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" layout="1column"
xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
<body>
<referenceBlock name="customer_form_login">
<action method="setTemplate">
<argument name="template" xsi:type="string">Rbj_Modulename::form/login.phtml</argument>
</action>
</referenceBlock>
</body>
</page>
Create a template file at location,
app/code/<Rbj/Modulename>/frontend/templates/form/login.phtml
Clear a cache and check your login page.
