This Mind Brain Ed blog article is an example of one that quotes the book Quiet kids: Help your introverted child succeed in an extroverted world. by Christine Fonseca in claiming that introverts and extroverts process neurotransmitters differently.
One of the main differences between introverted and extroverted brains is how they respond to the neurotransmitter dopamine. [...] Extroverts have more dopamine receptors and are thus more likely to seek dopamine-releasing stimuli.
Introverts seek the comfort provided by a very different neurotransmitter. While dopamine provides excitement and rewards, acetylcholine provides introverts with calm and relaxation.
Christine Fonseca (2014) explained that acetylcholine works the same for introverts as dopamine does for extroverts. The difference is that introverts feel happiest when focused inside.
I do not have access to the the Fonesca book; I cannot see if the book made this claim, or the references to support it. I can't find evidence. The psychology papers I've read seem contradicting. I don't know what to believe anymore.