We see more in aA New hope (film)Hope. Just as she records the message for R2 she also inserts a small disk/chip into one of his slots. With the plans not being found on the tantiveTantive it was indicated thus that they were transferedtransferred onto a physical medium.
With her inserting the object (looked like a card for a card reader if my memory serves right) into R2 I always took it that that wereit was the death starsDeath Star's plans that were being inserted into R2.