Hello Tushar
To use your custom login page:
1. Double-check your project settings to ensure that your custom login page is set as the default login page. Go to "Project" -> "Navigation" and verify that your custom login page is selected under "Authentication" -> "Sign-in page" or under "Default login page" and make sure that under "Role-based home pages" you select the correct home page for each user role.
2. Review your navigation structure to ensure that there are no links or references to the default login page.
3. Sometimes, browser cache can cause unexpected behavior. Try clearing your browser cache and then accessing your application again to see if the issue persists.
I recommend you read the https://academy.mendix.com/link/modules/183/lectures/1419/3.3-Custom-Login-Page on the Mendix academy.
It will take you through building your own custom login and much more.
And for reaching to login.html page, you can find the it by navigating to project/App from the toolbar -> "Show App Directory" -> "theme" -> "web" -> "login.html"