Got the answer from SO (https://stackoverflow.com/questions/56544823/whats-the-difference-between-mypy-iterators-and-generators).

Notice that:

• Iterators only have two methods: _next_ and _iter_ but generators have many more.
• Generators are a subtype of Iterators -- every single Generator is also an Iterator, but not vice-versa.

But what does this mean on a high-level?

Well, in short, with iterators, the flow of information is one-way only. When you have an iterator, all you can really do call the __next__ method to get the very next value to be yielded.

In contrast, the flow of information with generators is bidirectional: you can send information back into the generator via the send method.

That's what the other two type parameters are for, actually -- when you do Generator[A, B, C], you're stating that the values you yield are of type A, the values you send into the generator are of type B, and the value that you return from the generator are of type C.