when we have multiple reducers how is an action sent a reducer code example

Example: multiple reducers redux

/* Reducer Composition */
import { createStore, combineReducers } from 'redux';

const rootReducer = combineReducers({
  first: firstReducer,
  second: secondReducer
});
const store = createStore(rootReducer);

/* About Redux.combineReducers()
 * accepts an object in which keys are associated to specific
 * reducer functions, these keys are used as the name for the
 * associated piece of state
*/