A common question I hear is "How is saying 많아요 different than 많이 있어요?" And although I've answered it before in various comments, I've never made a video explaining it in more detail before.
많다 and 많이 있다 can both be used to say that there is "a lot" of something, and both are grammatically correct. But one of them might be preferred in some situations. I'll show you how they're different, when you might want to use one or the other.