My experience was the same as yours, resources explaining things assumed technical knowledge far beyond my level or were not very clear. This was a very well put-together article.
My only contribution to the discussion is that if you get a CORS error where you wouldn't expect it, the problem might not be a CORS issue. I spent the better part of a weekend trying to debug why a request to a Google API wasn't working and why I was seeing a CORS error (same thing worked fine on another system). Turns out, it wasn't the same thing, my url had a typo...
My only contribution to the discussion is that if you get a CORS error where you wouldn't expect it, the problem might not be a CORS issue. I spent the better part of a weekend trying to debug why a request to a Google API wasn't working and why I was seeing a CORS error (same thing worked fine on another system). Turns out, it wasn't the same thing, my url had a typo...