How to create new app has all erpnext modues then I can add more custom modules according to customer needs?
Could you please explain more precisely what you want to achieve?
If you need everything from erpnext, you can install erpnext and add custom modules in a separate app. One site can have multiple applications installed. In standard erpnext installation you already have 2 apps: frappe (it’s an app) and erpnext (which uses modules from frappe and adds additional modules).
so How to serve my custom app to the same site of erpnext app?
Create the app (there are a few tutorials, e.g. https://frappeframework.com/docs ), and install it to the site using bench. You may need hooks and fixtures. You can find more in tutorials and in erpnext forum (you can use google for searching - sometimes it is better than search function in discuss). Sometimes it’s faster to look in frappe, erpnext and custom app’s code in GitHub, than asking in this forum and waiting for answer. As a newbie (less than a year with ERPNext) I feel like the core team of ERPNext developers doesn’t like to help newbies, so I’m trying to search myself. It’s open source, so you have the possibility to copy some solutions.
just search forums and web, if there is no solution found, then open a thread