Det finns is used for something that exists rather permanently or exists as opposed to not existing.
Det är is used for more temporary descriptions.
I've added that. Note though that you can use morot the same way perfectly well in Swedish too, as a mass noun or whatever one would call it.
If I meant that there were something in the recipe and not a specific soup could I say det finns? There are onions in French Onion Soup is a little more permanent than there are onions in the (this) soup.
When it is singular: "Det är en morrot', the translation: "There is a carrot" would be not as accurate as "It is a carrot". Why in plural: "They are carrots" is marked as wrong.
In English, There is a carrot in the soup is proper. It is a carrot in the soup would be strange to say. You might say that if you were pointing out and identifying a whole carrot floating in the broth.
If there are several carrots in the soup, you could say There are carrots in the soup and it would be correct, but a native speaker would probably say There is carrot in the soup. Ingredients are generally considered an uncountable nouns, even if the raw ingredients started as countable things.
I disagree. If someone asked "what are those?" It would be perfectly fine for me, to say "they are carrots" or "they're carrots in the soup" (american english)
It is singular but are is only used with plural subjects. For it, it must be is.