"Intelligence" seems to consist of multiple different systems, but there are many tasks which recruit several of those systems simultaneously. That said, this doesn't exclude the possibility of a hierarchy - in some people all of those systems could be working well, in some people all of them could be working badly, and most folks would be somewhere in between. (Which would seem to match the genetic load theory of intelligence.) But of course, this is a partially ordered set rather than a pure hierarchy - different people can have the same overall score, but have different capabilities in various subtasks.

IQ in childhood is predictive of IQ scores in adulthood, but not completely reliably; adult scores are more stable. There have been many interventions which aimed to increase IQ, but so far none of them has worked out.

IQ is one of the strongest general predictors of life outcomes and work performance... but that "general" means that you can still predict performance on some specific task better via some other variable. Also, IQ is one of the best such predictors together with conscientiousness, which implies that hard work also matters a lot in life. We also know that e.g. personality type and skills matter when it comes to rationality.

I would suppose that the kinds of people referred to "the level above mine" would be some of those rare types who've had the luck of getting a high score on all important variables - a high IQ, a high conscientiousness, a naturally curious personality type, high reserves of mental energy, and so on. To what extent these various things are trainable is an open question.