The data is not stored in punch cards, the programs are/were. The data is/was stored in tapes or in tape emulators.
Storing data on cards would make random access wayyyy too slow (the operator would have to retrieve and load the card and keep them sorted manually, which is impossible to do at scale)
On tape, however, you can predict the distance you have to rewind by computing the physical length each records take if they have auto incremented IDs. A tape can contain a lot of records. Especially if it's a modern one or an array backed emulated one. At least that's what some people I went to school with told me it's how they did it.
I did a bit of stuff with data coming in on punchcards. You get a batch of punchcards, run it through a program (which can load data from tape or other punchcards) and a new batch of new punchcards with updated data come out on the other side, along with an impressive pile of printed invoices.
I doubt that physical media or tape are used beyond initial load onto disk, often directly into a database, with a single IEBGENER step. Heck, that was the case 30 years ago.
