> You have to have the unaltered originals somewhere, or you won't know what you hid where

You do not need originals.

Data can be encoded to look like noise yet still be decoded if you know the algorithm despite not having unaltered originals.

This is commonly done when secret messages are EM transmitted for example with turbo codes: https://en.m.wikipedia.org/wiki/Turbo_code

With stenography instead of encoding messages in the EM spectrum you encode in the media (sound, images, video, ...) you are using.

If you have data treated to look random (compressed / encrypted) you can for example encode it using the "least significant bits" of your media which are mostly sensor noise anyways.

A more sophisticated approach can spread this out across pseudo random offset pixels. Your algorithm knowing the pseudo random sequence can decode your data analogous to https://en.m.wikipedia.org/wiki/Spread_spectrum techniques for secret messages transmission and applications like: https://en.m.wikipedia.org/wiki/Low-probability-of-intercept_radar