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,

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.

Create a template file at location,
app/code/<Rbj/Modulename>/frontend/templates/form/login.phtml

Clear a cache and check your login page.