I wish more people read the CORS article on MDN[1] which helped me a lot at the time when I was trying to understand it. I knew some people had trouble with CORS but had no idea it was this bad, going by the comments here.
Exactly this. I can only upvote you once, but this is one article that should answer all the questions. Not just the simple origin case, but also how a preflight works.
[1] https://developer.mozilla.org/en-US/docs/Web/HTTP/Guides/COR...