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?

There is a raw score underlying any given IQ test that is an absolute value. It might just be as simple as the number of questions you get right. When testing a population, these scores form a normal distribution. We then scale the raw scores so that the mean/median or center of the distribution becomes an IQ of 100. So the raw scores can be compared across time and can vary, even though the IQ cannot as you said.