The relationship between a continuous signal (an image or wave function) mediated by light, to which photochemical film is exposed, is materially encoded on film in terms of random interactions between the random arrangement of silver in the film and the equally random arrangement of photon detections. The random aspect of both ensures a statistically neutral stance in terms of the otherwise continuous signal (wave function) being encoded or mediated. A side effect of this is localised grain or noise (god playing with dice) but which has no affect on the continuous signal (the image or the wave function). Statistically (ie. globally) the noise/grain cancels out. The image/signal/wave function occupies this same domain (ie. where the noise cancels out). But the materials (photons, silver, etc) don't. They occupy a local domain, no less visible, but producing that effect we call grain or noise.
In film there is the opportunity for very fine details being mediated because the randomness is pure randomness, ie. statistically neutral and at all scales (down to subatomic levels). But the cost of such is noise or grain. There's a particular cost/benefit tradeoff going on in this process.
Now if you start with a digital signal, the signal is discontinuous or quantised. There's a different sort of tradeoff going on. The arrangement of materials is such that it sacrifices neutrality at lower scales in exchanage for less localised noise and a more computationally manageable information system. Now a digital signal (image) occupies the same domain as a continuous signal/image, but accordingly it is likewise unaffected by any localised noise. Adding noise (be it synthesised or through prior capture of neutrally exposed film, etc) does not affect the digital signal. Just as it does not affect the continuous signal. The noise is just the visibility of the localised variations. These localised variations are easy to interchange between digital and film methods. It's the signal (the image) which fails to be affected by such interchanges.
In other words, if you look "through the grain" of a digital signal (to which grain was added by whatever means) you'll just see the original digital signal. No different from what it was in the first place other than it might be harder to see. And in film, if you look "through the grain" then you'll see the underlying non-digital signal: the 'analog' (or continuous signal). If likewise harder to see.
The grain or noise is just a localised phenomena. The image proper, digital or otherwise, remains lurking there on the other side of the haze, so to speak. No different from how it began.