Where does the uncredited information for crew members on old films originate?

Define "old" please.

Perhaps the URL of an example?

Thanks for the input, but I maybe didn't phrase my question in quite the right way. Let's take the 1965 film of Genghis Khan as a random example. The full crew list gives a whole host of names that are apparently uncredited on screen: https://www.imdb.com/title/tt0059219/fullcredits?ref_=tt_cl_sm#cast  

My question is basically where does this kind of info come from? Given the age of the film (and there are plenty of examples from the 50s & 60s that I've seen), I doubt whether all of the crew members are updating their own database entires (most of the crew will be at best retired at worst no longer with us, I suspect). So I'm assuming that there must be a full list of who was on the payroll for a given film that was used as the basis for the database entry. And that is what I'm trying to get at: where does that information exist, and how did it get into imdb?

So, say I was researching a film, or a crew member, and I wanted to verify that they did, in fact, work on film A that lists them as uncredited, where would I need to go to access that information? The studio archives? The production company archives? Or…??

The same would also be true if I wanted to find the shooting dates, as opposed to the release dates, for a production. Any pointers gratefully received.

Thanks, Karen,

I can see that this is all going to be more complicated than I'd hoped. Although I take your point about individuals posting updates, I'm not quite convinced that this is the main source for uncredited credits (the Genghis Khan example, for instance, features a large number of uncredited crew and it would be a significant coincidence if all of them had updated their profiles, not least because I know at least one of them died before imdb came into being).

Fundamentally, though, your explanations suggest that imdb is more crowd-sourced than I'd imagined, which potentially puts it on a level of trust equivalent to wikipedia, ie a starting point but not a verifiable source of truth. Clearly, I need to dig a lot more into the sources for various productions/studios to see what archive material exists where…