Create a layout file vendor_name/module_name/view/frontend/layout/customer_account_login.xml
replace template
<?xml version="1.0"?>
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
<body>
<referenceBlock name="customer_form_login">
<action method="setTemplate">
<argument name="custom_template" xsi:type="string">vendor_name_module_name::custom_template.phtml</argument>
</action>
</referenceBlock>
</body>
</page>
add new content using container customer.login.container
<?xml version="1.0"?>
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
<body>
<referenceContainer name="customer.login.container">
<block class="Magento\Framework\View\Element\Template" after="customer.new" name="custom.template" template="vendor_name_module_name::custom_template.phtml"/>
</referenceContainer>
</body>
</page>
add new content using container form.additional.info
<?xml version="1.0"?>
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
<body>
<referenceContainer name="form.additional.info">
<block class="Magento\Framework\View\Element\Template" name="custom.template" template="vendor_name_module_name::custom_template.phtml"/>
</referenceContainer>
</body>
</page>
add new content using container content
<?xml version="1.0"?>
<page xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="urn:magento:framework:View/Layout/etc/page_configuration.xsd">
<body>
<referenceContainer name="content">
<block class="Magento\Framework\View\Element\Template" name="custom.template" after="customer.login.container" template="vendor_name_module_name::custom_template.phtml"/>
</referenceContainer>
</body>
</page>