This is something that people often don’t know about. For certain things there can actually be little to no experts. One example, ski lifts. There are only a handful of people in the entire world who know how to splice together ski lift cables.
A more concerning one is nuclear engineers. There’s been such a stigma against nuclear power that the amount of people who know how to build a nuclear reactor has fallen to incredibly low numbers. Also, the US had to reverse engineer some of their own nuclear weapons because the people who built them all died and the knowledge of how they were built died with them.