Is it okay to edit built-in doctypes scripts or we should create a copy and then edit in it?
More details will helpful.
You can use
Edit 2022-01-13: Updated links
You can edit the doctypes and make the customization but then you will have to maintain the repo for the frappe/erpnext but you might get the conflicts while updating or upgrading.
A better option would be to use the Custom Script.
Update 2022-01-13: Client Scripts
But how can we write python part in custom scripts?
I need to get data from other doctypes for which we need to edit doctype.py and doctype.js files in the server as shown in the scripting tutorial by erpnext.
Also if I add some extra custom fields to the built-in doctypes, will they also create conflicts while updating or upgrading? Or they will be overwritten?
Refer to the link for more info.