Partly but also by effort and upbringing

You got be lucky to be raised in the right upbringing. One's upbringing is not their choice.

Effort? I have yet to see any data indicating one can raise their intelligence^1 by effort. What I personally believe is that effort, hard work, tenacity, etc. are all just metaphorical raffle tickets in a metaphorical "success" raffle. Effort is required to play the game, but it at best can only increase your chances of winning. After all, it is by no means a guarantee. More effort = more tickets/chances. Good upbringing? Generational wealth? All more tickets. But, you hardly put in any effort, have a poor upbringing, and still win, but the odds are stacked against you.

[1] At least intelligence in how it's defined and measured by psychology. I do not necessarily agree with psychology and its tools of measurement, but that is irrelevant.

If you took your definition of "smartest" person and never taught them to read , write or do math they wouldn't be very intelligent. Intelligence is definitely learned.