Is it plausible to create a world in which the people who live inside it developed technologies like the ones we had in WW2, but after their progress came to a halt they forgot their knowledge. Meaning that they still have their weapons, tools, etc. from that time but don't know how they work, scientifically? Could it be from a nuclear apocalypse?
Example: Knowing how to drive a car but not knowing how they work.
It seems that what you want is something that retards human problem-solving skills. This can be done with disease or toxins.
Exposure to certain types of disease or toxins. Diseases like whooping cough, measles, or meningitis can cause intellectual disability if medical care is delayed or inadequate. Exposure to poisons like lead or mercury may also affect mental ability. (Source)
Difficulties with the idea
Said another way: if you take away our ability to understand the technology behind a car (gears, friction, leverage, combustion, electricity, etc.) you also take away high-volume farming, large density water distribution, disease control, etc. The underlying technology used to maintain society at the population densities enjoyed during WWII would very quickly turn cities into death traps (starvation, sanitation, disease).
Said in yet another way: Losing the ability to design, repair, and maintain a car means losing the ability to understand why you'd use the car in the first place. You wouldn't solve the problem of getting from A to B by thinking, "I can use a car to do that!" You'd simply walk there, having never considered the car as a possible solution to the problem.
It takes time. Nothing can retard brain functions in a moment, or even in weeks. It may take months of lead exposure to begin affecting a child, but it would take years to begin affecting a trained adult. Why? Because cognition is very complex, involving memory, sensory processing, abstraction and association, etc. A great deal of adult behavior has transitioned from learning to do something to the habit of doing something. This is why people with forms of dementia can drive cars (they may have no idea where they ended up, but they successfully operated the vehicle). This is because the skill of driving has become so habitual that it doesn't require nearly as much thinking as it does when learning how to drive.
Radiation isn't selective. Radiation strong enough to destroy the brain's higher functions is destroying the rest of the brain with them, including its ability to manage autonomic functions like breathing and blood circulation.
I believe forced mental retardation of a large population is plausible from the perspective of an engineered disease so long as you remember that such retardation will manifest in specific ways with very general results. Here's a few off the top of my head.
I think that is entirely possible. It likely needs to be from some apocalypse that pretty much shuts down education. That doesn't have to be nuclear, it could also be global warming melting all the icecaps and making much of the world inhabitable, it could be a religious know-nothing apocalypse (religion takes over that rejects "science" as evil because it creates doubts about scripture, for example).
You fictionally need to destroy the modern engineering college education, because that is how the next generation actually learns about creating machines, semiconductors, computers, etc. Up through about the Master's degrees. Then the PhDs go on to discover new principles, materials, etc to create NEW devices and machines.
If you stop the education around the middle school level, you might have some that can fix simple cars and engines, but nobody would know how to fix modern computerized cars and engines.
I don't think so. Almost all technology requires maintenace, and that ultimately requires understanding.
You could have a driver of a car who does not understand how it works. He or she might be taught by rote to change oil every now and then, but once the last oil can on the shelf is sold somebody has to know how to refill it. Oil is something like gasoline, and there is still gasoline in the storage tank. Oops, scratch one car.
Try something easier, a bicycle. Patch kits for the inner tube are easy to use, and they might even come with a little instruction booklet. But what if the last pre-cut patch is used, if the last tube of glue is dried out? Or worse, what if the tire is worn out? You would need somebody who can manufacture new spare parts.
All you have to do is stop one generation. Kill off everyone over the age of 5. Or kidnap all children and have them raised by people who don't teach them anything about tech. Then kill off the older kids and adults left behind and reintroduce the kids (who can be grown up now, even with their own kids) into the communities they came from, which are now empty of people. (This can be done as cultural genocide, as saving kids from a disaster, or many other ways...the reasons aren't as important to this part of the question as the results.)
Some people will figure out how the tech works. That's just how it goes. But no one will understand it before they return to the places where it exists.
So much knowledge can be lost in just one generation. This has, sadly, happened many times in human history (think native peoples in Canada where children were taken from their homes and not allowed to speak their own languages or follow their customs; also happened in the US and Australia). Not for tech per say but still, loss of cultural and institutional knowledge.
It sounds to me like people have kinetic skills/knowledge but less of the (abstract) knowledge based skills.
This could happen pretty easily if everyone was unable to pass on their 'knowledge', but can still show someone how to do something (drive a car, do a oil change, use a tool etc.).
I think a disease (bio-weapon) which affected peoples ability to read would do the trick.
Perhaps, a disease which either affects some part of the visual cortex. 'Contagious dyslexia'. Or even a disease which causes farsightedness in almost all children, making it too difficult to teach a generation to read.
We could get a handle on the disease after a few generations, but by that time the amount of people who can read has dropped below a level which society can sustain its (now) esoteric use.
The largest problem with keeping technology in working condition is maintenance. Stuff breaks with use (even disuse) and that requires people knowledgeable enough about the inner workings of a machine's mechanism (not to mention being able to acquire the right raw materials) in order to perform repairs.
This was quite true with WWII technology and is still quite true today. You'd have to advance technology enough to make machinery that is self-repairing and/or self-duplicating without any human intervention. At this point people only have to know how to use the machine without knowing how it works. Eventually, as generations go by, the people who knew how the machinery worked die off and might not pass on their wisdom to the next generation. Libraries and technical manuals would also have to be lost and/or forgotten. (Perhaps as a result of natural and/or man-made disasters.)
There were several examples of this in the Stargate SG-1 TV series where Earth's humans were trying to acquire advanced enough technologies to defend themselves even if they didn't quite know how to reproduce it. Nevertheless, a notable example of this can be seen in The Sentinel, where a "primitive" agrarian society was in possession of a super-powerful weapon built by their ancestors. The machine broke, but no one native to the planet knew how to fix it as that information was long lost and forgotten.
Certainly, as demonstrated by Asimov in his major book series Foundation (Wiki article). The premise is that a Galactic empire was in decline, largely because they had abandoned primary science and just focused on maintaining technology. Couple that with economic stagnation, political corruption and the occasional wars and decline of knowledge seems probable. It's not that the books didn't exist explaining things, but rather the generations of people who understood the explanation died off, leaving mechanics.
Hate to say it, but one easy way to do this from a stroytelling perspective is to drastically reduce the population. I'm talking either everyone but a certain set of like-minded people (no one is a specialist), or else you've got to leave around less than a hundred people alive. This loss of knowledge could be temporary if so desired, as people can teach themselves things.