In addition, the factory provides a list of resignation procedures, which is very troublesome and requires a lot of signatures. I suggest you start the formalities the day before they approve your resignation date, otherwise it will be too late the next day. For example, if they approve your resignation on the 20th, then you should start to find someone to sign on 19, otherwise you will have to find someone for half a day in such a big factory.
As for writing the reasons for resignation on A4 paper, just tell the truth. I have read many resignation letters written by them, and they are all honest. Many people say that they will go back to apply for certificates and exams. You can also refer to the same.