The higher IQ of developed countries is solely because of prenatal/early childhood nutrition. So long as a child is literate and has a relatively ok knowledge of basic vocabulary, IQ tests are generally very accurate. You cannot study for IQ tests; it's a well-known thing that rich parents often try and fail to get their kids tutored for the IQ test.

Obviously, I agree with you about the requirement of IQ testing for certain jobs (if it ever happens, I've never heard of it but it sounds insane).

I and most of my family are "gifted." My uncle is classified as "retarded." His IQ is 69, he just barely made the cutoff to receive assistance. But, that doesn't mean that he's less valuable as a person; he's fun to be around, and likes to play fortnite and go fishing with my little brother. He has a job through a special program, and (as far I know) is genuinely happy with his life. However, he definitely could not be a scientist. He can't understand the concept of math or time, like Thursday being farther away than Wednesday type thing.

Obviously he's an extreme example, but it kind of translates up the spectrum; IQ doesn't change whether or not you can live a fulfilling life, but it does make it harder to go into certain professions.