Usually richer people get trained on a language through school so they learn to pronounce stuff "correctly". Thus poorer people pronounce it incorrectly and eventually they both have accents.

If you send settlers somewhere without anyone teaching them the "proper" language eventually their language will evolve differently. They'll shorten words. Emphasize certain letters and so on.

You know how you have a voice that helps you talk and say things? Well, different people also have voices, but they are not the same as yours. Some people say things differently than you do, even if they use the same words. This is called having an accent. Accents are like different ways of singing the same song. For example, you can sing “Twinkle Twinkle Little Star” in a high voice or a low voice, or a fast voice or a slow voice. These are like different accents. Now imagine that different people live in different places, like different countries or cities. Sometimes these people don’t talk to each other very much, so they start to sing their songs differently. They may change some of the sounds or words in their songs, or they may make up new songs. These changes make their accents different from each other. For example, people who live in Australia may sing their songs differently than people who live in Britain or America, even if they use the same language. That’s why their accents are different.

You ever had a group of close friends, where over time you develop in-jokes and anecdotes together until one of you can say "umbrella!" and you all break out laughing while everyone else thinks you're crazy?

Humans form bonds and shorthands together. Keep a group of humans in relative isolation for a long time and they'll personalize their own method of communicating with each other.

That is a part of it but a very small one compared to other aspects. For the most part is it because languages mix a bit when moving.

The people occupying america before the british got there spoke different languages, and a lot of the people that moved there during the colonies and helped shape the region also spoke other languages. The people growing up in these areas heard different pronunciations and for whatever reason some stuck and some didn't. Rich and poor people do sound different, but that is just part of the local differences, and it is mostly because of the people who surround you rather than anything else.

Australian accent is very rooted in the older british accent because they were a colony, but they also had some mixing and ended up with some different consonants. American accent also rooted in british but also affected by french phonemes and in some areas even phonemes from italian and spanish.

You can also notice scottish and Irish accents have very noticeable phonetics because the original people of those regions spoke languages that used very different phonemes than the rest of western europe, their accents are remnants of their origins.

Regional accents within countries also exist, and are usually traceable to immigration or foreign cultural bubbles. Think new york accent being influenced by italians, or how we use different language words for live animals and their meat.

'jomocda' is my in-joke with my friend and i dont even remember where it comes from

theres also evidence that the "american" accent is also the original british accent but when people starting coming to america the british accent changed over time while they kept the original accent