Can you be a bit more specific about what you’re trying to do? If you just need to run some code in response to a document event (save, submit, delete, etc.), the easiest approach would probably be to just use server scripts (no app needed): https://frappeframework.com/docs/v13/user/en/desk/scripting/server-script
What I want to do is over ride a few features in Work Order like:
When a user clicks on Start button I want to automatically insert Material Transfer entries.
In the same way when a user clicks on Finish button I want to first ask the user to enter Qty Manufactured & Qty Rejected, update the Work Order and then insert necessary Material Transfer entries.
If all the quantity of the item are manufactured then I want to automatically update the Qty to Manufacture in all the Work Orders following current Work Order.