let’s say that i have two decks. deck one has field A and B, and deck two just has field B. i create a field A for deck two, but it is blank. how can i copy the content from field A in deck one into deck two, by matching the field B between the two note types?
I would do this by importing a text/CSV file of the notes that made the “deck one” cards to update field-A in the notes that made the “deck two” cards. Since field-B will be what matches between the sets of notes, that should be your first field. With the right Import Options selected, Anki will find the matching notes, and import field-A into them.
Experiment with a small set of notes first, to make sure you’re getting what you want. Make a fresh backup of your collection in case you want to restore that – and don’t forget you can Edit > Undo most actions (even import).