Where does useSelector Pull State from if it is Not Passed In?

If you look at the index.js you should see a <Provider wrapping the App component as so

<Provider store={store}>
  <App />
</Provider>

The useSelector hook gets the state from the redux store sent in the wrapper <Provider component.

The Provider makes the Redux store available to the component hierarchy below.

I'm a Redux maintainer and author of the last couple React-Redux versions.

Internally, useSelector gets access to the Redux store via a useContext hook. It then calls store.subscribe() to be notified any time an action is dispatched, and uses store.getState() to get the latest state. Finally, it calls yourSelector(state).

See my extensive post The History and Implementation of React-Redux and talk ReactNext 2019: A Deep Dive into React-Redux for more details on how React-Redux works internally.