A movie takes a lot of memory unless you can find some way to compress all those pixels. One way is to start with the initial frame. That’s a lot of bits, but when people move at normal speeds the image of the next frame is almost the same as the first one. All you really have to do is record the pixels that are different, which is usually a huge savings. The next frame you treat the same way, and so on, until the changes add up to enough where starting fresh with a complete new frame is reasonable. (There are other aspects and I omit a lot of details I didn’t bother to look up.)
The MIDI protocols for encoding music encode not the sounds but the characteristics of the note (and instrument): pitch, volume, duration, vibrato, etc. When you feed the info to the playback system you don’t so much reproduce the original sound as reconstruct it. You could send the note info to a different “instrument” if you cared to; irrelevant details would be ignored and missing characteristics replaced with defaults.
Another simplification that reduces storage is to filter out unimportant details, perhaps replacing them with a generic object or leaving them out entirely.
And when storing data you can keep track of the less frequently used stuff and put it in slower storage. I read somewhere that the ancient Chinese imperial records got so unwieldy that they condensed them. And a few centuries later, did it again. Early records got a bit sketchy in the process.
I think you see where this is leading. When I try to remember things from Liberia, where there are pictures to compare to my recollection, I find that I often have what amount to “stills” of a scene in mind as background to the action, and the rest goes on in front like Yogi Bear’s head. And the “still” doesn’t match the photograph very well. The tree branches that we climbed on are clear enough, but those we didn’t handle seem to have been mentally replaced with “more tree”. I abstract the bejabbers out of the scene—sometimes including the dialog too. Yet other dialogues I recall word for word.
There seems to be a differential discriminator at work for medium-term storage. A match flaring is a big change; that gets filed. The puffs of flame scurrying around the hot coals later—not such big changes, not filed. Even though the match was 3 seconds and the hot coals were an hour, I remember the match more easily.
But the things we don’t remember still have some effect on us. I’ve heard amnesiacs can have “muscle memory” for things they can’t recall or sometimes even name. There was a very interesting report on someone whose short-term memory wasn’t there anymore, but slowly learned some mental tricks anyway. I know no way of measuring this effect, but ... My father had dementia and lost a lot of his memories, but those who worked with him said he was always a gentleman. A lifetime of actions built something that wasn’t conscious.
It seems that we don’t think things are real unless we consciously remember them (do people think they’re more real if they’re on TV?). That skews our view of our lives to only the dramatic (and usually unpleasant) incidents. But there was more to our lives than the flickerings on the memory screen.