I am creating a login page where in currently email id and password are being displayed one below the other but i need a setup in which user enters the email and clicks on next and in the backend it checks if user email exists then ask for password or else prompt an error "Email id not signed" .
to accomplish this you need either a third party app or a custom private app. That info cannot be accessed via the front-end directly.
Whatever solutions, this needs custom development by a Shopify Profesional/Expert.
We can help with this if you are interested.
Sorry, no idea.
What I meant in my message is that checking if a customer exists has to be done from an app, either public or private, but I do not know about any app that does this.