American IQ scores have rapidly dropped, proving the 'Reverse Flynn effect'

Can someone actually explain how IQ tests work? By work, I mean how are the tests engineered, and the results computed.

Long time ago someone explained to me that the engineering of IQ tests was actually drafted from a very large pool of (regularly updated) questions, where statistical significance was extracted to form a _core symposium_ of questions to sample from. Also, the IQ score itself was normalized to be normally distributed centered at 100.

With this understanding, I was under the impression that IQ was a relative measure, at a specific point in time, of one's placement in the distribution.

Which meant to me that IQ cannot "drop" across a population, the mean will always be 100. And IQ scores cannot be compared on a time series basis, since they are only cross sectional measures.

Is that all wrong? Is there some truth to it?

My understanding at a high level is that an IQ test is basically made by generating a bunch of cognitive tasks that putatively test different aspects of cognitive functioning (e.g. verbal reasoning, visuospatial reasoning, attention, working memory, etc.). For most people, performance on one type of cognitive task (e.g. verbal reasoning) is highly correlated with performance on the other types of cognitive tasks.

This allows you to model the test as a hierarchical statistical model with some general intelligence factor (denoted G) at the top and then specific cognitive tasks branch off from there. You can then infer what G is just by statistical inference on the "branches" (the performance on the individual cognitive tasks); similar to how you might infer someone's height if you only had access to their leg and arm lengths, as these are highly correlated with each other and also with height.

I believe IQ scores are always population normed to have a mean of 100 but unnormalized scores are likely available to compare across time.