Mode of Saving Data in Datasheet / Firestore

I don’t quite understand how the Save Data to a Datasheet / Firestore is supposed to work? There are 3 modes: Auto, Add, Update and I am assuming that if one uses Auto it should decided if it has a new key which then needs to be added or if it has an existing key which then needs to be updated. If this is correct then I think this only works if the source datasheet is the same as the target datasheet. I tried to do this with different datasheets and used a list with a button component. If the user clicks on the button it needs to save the chosen row to a new datasheet and then shows a new screen. If the user then navigates back to the previous screen and choses a different option it will create another new record. I assume it does not update the key record because the datasheets are not the same. In this case it always adds a new record even if the key field value is the same. I am not sure if this is a bug or it was supposed to work that way?

There is also the field Action ID which I don’t understand how it can be used. I couldn’t find any thread/video about it.

Neonto, could you please explain how this should work?
Thanks