For call the Login form in another PHTML file. Please use the below code:
<?php echo $this->getLayout()->createBlock("Magento\Customer\Block\Form\Login")->setTemplate("Magento_Customer::form/login.phtml")->toHtml();?>
For call the Login form in CMS Page/Static Block. Please use the below code: {{block class="Magento\Customer\Block\Form\Login" template="Magento_Customer::form/login.phtml"}}