How does redux store works behind the scenes

redux.js

/*
 * reducers/user.js
 */
const userInitialState = {
  authToken: null,
  userId: null
}
const userReducer = (state = userInitialState, action)=>{
  return state;
}


/*
 * reducers/app.js
 */
const appInitialState = {
  currentView: 'home',
  userId: 1
}
const appReducer = (state = appInitialState, action)=>{
  return state;
}


const combineReducers = (state) => {
  return Object.keys(state).reduce((previous, current) => {
    previous[current] = state[current]()
    return previous;
  }, {});
};

/*
 * store.js
 */

// Built in redux functions
const createStore = (reducer, enhancer) => {
  const store = Object.assign({}, reducer);
  return enhancer(store);
}

const applyMiddleware = (...middlewares) => store => {
  return middlewares.reduce((previous, middleware) => {
    return middleware(store)
  }, {});
};

// Mock middleware
const logger = (store) => { 
  console.log("LOGGER:", JSON.stringify(store));
  return store;
};

// what we actually type in the store.js file
const store = createStore(combineReducers({user: userReducer, app: appReducer}), 
                          applyMiddleware(logger));

// Reduced store
console.log(store);