It is a fake latent variable cardiologists (who were being paid by tobacco companies) came up with to distinguish between people who had heart attacks and those who didn’t to mask the effect of smoking.
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3477961/
It’s not surprising that this vocabulary persists - it’s a nice shorthand to casually describe personality. But the idea that it represents anything objective, measurable or scientific needs to be stamped out.
Our mind is more complex than simple dichotomies like this assume. We all behave like "introverts" in some situations and like "extroverts" in some situations. We constantly adapt to our environment and our responses to external stimuli are not stable in time. They also depend on our internal state like hunger, tiredness, etc. much more than these sort of classification schemes would have you believe.
This is true, but it's also true that peoples' average preferred amounts of stimulation are very different to other people's. And those averages often are quite stable.
As for introvert/extroversion: On Susan Cain's book "Quiet"
The "Galen’s Prophesy" reveals certain behavioral predispositions of children within four months old: https://www.youtube.com/watch?v=CVJBzvaylH8&ab_channel=JillB...