How it can find following url route? and where it will get backend code? if we use " / "

login.call = function(args) {
$(’.btn-primary’).prop(“disabled”, true);

$.ajax({
	type: "POST",
	url: "/",
	data: args,
	dataType: "json",
	statusCode: login.login_handlers
}).always(function(){
	$('.btn-primary').prop("disabled", false);
})

}

I mean in which file routes are defined ?

Use /api/method/route.to.python.module

please read:

https://frappe.io/docs/user-guide/rest_api

Section on RPC