I want to make a custom field and the functionality of that field with the custom app, How can i do that without change the erpnext code only using custom app ?
The changes will definitely change code in erpnext app. However, if you wish to ship your changes use the hooks to add fixtures and export.
Can you give a example of it or any reference ?
i want to make it those changes from my custom app like add search customer by email field and when someone give the email address then customer field will be automatically fetch that customer or throw error like no customer exists for this email address.
Use create_custom_fields function if you need them created dynamically. You can also call this during install by adding it in hooks.py
Example: erpnext/shopify_settings.py at 4128aa762887ff7a82621420c7f4c8bfa637de76 · frappe/erpnext · GitHub
