IQ is heritable and it can’t be fully explained by wealth. Twins raised in different families have a high IQ correlation between them whereas non-twin siblings raised in the same household have far less IQ correlation. This suggests that IQ is likely affected by some combination of genetics and the maternal womb environment much more so than the household environment (where wealth would play a big factor).

Now, the maternal womb environment is not wholly uncorrelated with wealth. Maternal health including lifestyle, activity, and nutrition may play a big role here. Since all of those are positively correlated with wealth, this suggests a potential cause (which still needs to be investigated) of maternal health on fetal brain development and therefore IQ.