Hi @Anonymous ,
Could you please share more details about your scenario? What data would you like to copy from list A to list B? How do you know which item of list B should be updated when clicking a button which belongs to a specific record in the Gallery, depending on what relationship? Suppose there are two items in the gallery, there will to two buttons in the gallery belong to respective items.
If there is a relationship, you can update list B in the App directly using Patch function:
Patch(ListB, LookUp(ListB, ID = ThisItem.Id),{ColumnToUpdate: ThisItem.ColumnName})
So the point is you need to explain more on the two lists.
Best regards,