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I once did one of those quizzes that figures out where your American accent is from and I got mostly LA and midwest. Makes sense since I learned from watching TV shows.
Wait how does that work?
There's many regional differences in American English.
First, pronunciation is always changing, and changes tend to happen regionally.
For example, there's the Mary-merry-marry merger. A bit over half of American speakers pronounce all three of those words identically, as mɛri. About 17% of Americans have a full three-way contrast. In NYC, for example, they'd say meɹi, mæɹi, and mɛɹi. And other people merged two of the three.
The pen-pin merger is a famous feature of southern American dialects.
Some words have regional pronunciations - crayon can have one or two syllables, for example.
And then there's regional words, like pop vs soda, bucket vs pail, firefly vs lightning bug, you vs y'all vs yinz vs youse vs you lot vs you all vs you guys etc.
By asking about all of those sorts of things, you can figure out where someone's from.
Dialect tests. Think about how someone from boston might say "park" like "pahk" vs other parts of the country, or if someone uses "y'all" where they might be from. The way people pronounce o,a, ai, ough, augh type of sounds is very telling. Also phrases are very regional. There are many studies that compile that data. One famous dataset is used in a Times article that is behind a paywall, here are some people talking about it: https://peabodyawards.com/nytimesdialectquiz/
Another random one from buzzfeed: https://www.buzzfeed.com/andrewziegler/dialect-quiz
And babbel: https://www.babbel.com/en/magazine/american-accent-quiz
Or just search for dialect quiz.
The Buzzfeed one got where I've lived most of my life. Wasn't sure where it would say since I moved around a lot as a kid.