When you visit the URL /login, the browser makes a GET request for the URL. This request hits the Sessions controller's new action, which returns a view displaying the login page.
In the login form, we use form_for(:session, url: login_path) do |f|. This refers to the name of the resource and corresponding URL. In the signup form, we used form_for(@user) do |f| since we had a User model. For the login form, we don't have a Session model, so we refer to the parameters above.
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