How to override login.phtml template in Magento 2?

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.