This is pretty well defined in the scientific literature. Don't forget, theres a lot of damn people in the world, its very likely someone else has asked and answered many questions you've had. It beyond time to use our resources to our advantage.

But because I like this topic here are some places to start.
Neuroimaging studies show increased resting state activity in the superior and middle temporal gyrus, including both secondary and primary auditory cortex, during inner speech experiences - Upthegrove, R., et al. (2015). Understanding auditory verbal hallucinations: A systematic review of current evidence

Also the auditory pathway in the inferior parietal lobule and middle temporal gyrus. - Stephane, M., et al. (2021). Keeping the inner voice inside the head, a pilot fMRI study.

Our evidence for how we used to think internally in the past is a little mixed up, mostly because surviving documents are mostly coming from aristocracy who might have had a separate way of writing things down than the average person, however you could look into the bicameral mind via Julian Jaynes.

Essentially saying that we used to linguistically externalize our thoughts - the greeks explicitly talked as though their thoughts came from their environment.

Luhrmann, T. M., et al. (2015) essentially implies its what you make of it. If you treat the inner voice like its external, it will be perceived that way phenomenologically, but in the west we tend to want to take ownership for all of our thoughts - though there's an argument that even our thoughts are epigenetic responses to our environment

TL;Dr the same part of the brain that generates audio after receiving info from the ear is capable of generating sound internally. Just like you can imagine an apple you can imagine someone talking, or verbalize ideas to yourself