How to copy document field data in firebase firestore and save it to sub collection document?

I have a firebase firestore database and there's a collection called collection1 where I need to copy each field of the document in my collection1 and then save it to new collection document that is a sub collection of collection1 document. I need to do this once a week eg. Sunday 11:59 PM

I just don't have any idea on where to start or how am I supposed to do it.