①configureStore関数の実装

node_modules/@reduxjs/toolkit/src/configureStore.ts

/** 116行目 */
export function configureStore<
  S = any,
  A extends Action = UnknownAction,
  M extends Tuple<Middlewares<S>> = Tuple<[ThunkMiddlewareFor<S>]>,
  E extends Tuple<Enhancers> = Tuple<
    [StoreEnhancer<{ dispatch: ExtractDispatchExtensions<M> }>, StoreEnhancer]
  >,
  P = S,
>(options: ConfigureStoreOptions<S, A, M, E, P>): EnhancedStore<S, A, E> {
  const getDefaultMiddleware = buildGetDefaultMiddleware<S>()

  const {
    reducer = undefined,
    middleware,
    devTools = true,
    preloadedState = undefined,
    enhancers = undefined,
  } = options || {}

  let rootReducer: Reducer<S, A, P>

  if (typeof reducer === 'function') {
    rootReducer = reducer
  } else if (isPlainObject(reducer)) {
    rootReducer = combineReducers(reducer) as unknown as Reducer<S, A, P>
  } else {
    throw new Error(
      '`reducer` is a required argument, and must be a function or an object of functions that can be passed to combineReducers',
    )
  }

/** 以下略 */

以下の部分でcombineReducers関数を呼び出し、Reducerの初期化を行っています。

node_modules/@reduxjs/toolkit/src/configureStore.ts

  } else if (isPlainObject(reducer)) {
    rootReducer = combineReducers(reducer) as unknown as Reducer<S, A, P>
  }

以下でcombineReducers関数をimportしており、combineReducers関数がreduxライブラリの関数であることが確認できます。

node_modules/@reduxjs/toolkit/src/configureStore.ts

/** 1行目 */
import type {
  Reducer,
  ReducersMapObject,
  Middleware,
  Action,
  StoreEnhancer,
  Store,
  UnknownAction,
} from 'redux'
import {
  applyMiddleware,
  createStore,
  compose,
  combineReducers,
  isPlainObject,
} from 'redux'

/** 以下略 */

②combineReducers関数の実装

node_modules/redux/src/combineReducers.ts

/** 123行目 */
export default function combineReducers(reducers: {
  [key: string]: Reducer<any, any, any>
}) {
  const reducerKeys = Object.keys(reducers)
  const finalReducers: { [key: string]: Reducer<any, any, any> } = {}
  for (let i = 0; i < reducerKeys.length; i++) {
    const key = reducerKeys[i]

    if (process.env.NODE_ENV !== 'production') {
      if (typeof reducers[key] === 'undefined') {
        warning(`No reducer provided for key "${key}"`)
      }
    }

    if (typeof reducers[key] === 'function') {
      finalReducers[key] = reducers[key]
    }
  }
  const finalReducerKeys = Object.keys(finalReducers)

  // This is used to make sure we don't warn about the same
  // keys multiple times.
  let unexpectedKeyCache: { [key: string]: true }
  if (process.env.NODE_ENV !== 'production') {
    unexpectedKeyCache = {}
  }

  let shapeAssertionError: unknown
  try {
    assertReducerShape(finalReducers)
  } catch (e) {
    shapeAssertionError = e
  }
  
/** 以下略 */

以下の部分でassertReducerShape関数を呼び出し、Reducerの初期化を行っています。

node_modules/@reduxjs/toolkit/src/configureStore.ts

  try {
    assertReducerShape(finalReducers)
  } catch (e) {
    shapeAssertionError = e
  }

以下でassertReducerShape関数を定義しています。

node_modules/redux/src/combineReducers.ts

/** 62行目 */
function assertReducerShape(reducers: {
  [key: string]: Reducer<any, any, any>
}) {
  Object.keys(reducers).forEach(key => {
    const reducer = reducers[key]
    const initialState = reducer(undefined, { type: ActionTypes.INIT })

    if (typeof initialState === 'undefined') {
      throw new Error(
        `The slice reducer for key "${key}" returned undefined during initialization. ` +
          `If the state passed to the reducer is undefined, you must ` +
          `explicitly return the initial state. The initial state may ` +
          `not be undefined. If you don't want to set a value for this reducer, ` +
          `you can use null instead of undefined.`
      )
    }

    if (
      typeof reducer(undefined, {
        type: ActionTypes.PROBE_UNKNOWN_ACTION()
      }) === 'undefined'
    ) {
      throw new Error(
        `The slice reducer for key "${key}" returned undefined when probed with a random type. ` +
          `Don't try to handle '${ActionTypes.INIT}' or other actions in "redux/*" ` +
          `namespace. They are considered private. Instead, you must return the ` +
          `current state for any unknown actions, unless it is undefined, ` +
          `in which case you must return the initial state, regardless of the ` +
          `action type. The initial state may not be undefined, but can be null.`
      )
    }
  })
}

/** 以下略 */

下記の通り、オブジェクト内のreducerに対し、init処理を行っていることが確認できます。

node_modules/redux/src/combineReducers.ts

  Object.keys(reducers).forEach(key => {
    const reducer = reducers[key]
    const initialState = reducer(undefined, { type: ActionTypes.INIT })

おわりに

storeにreducerを渡しただけで使えるようになる、というredux-toolkitの仕様にやや困惑したので、ソースをさかのぼって確認してみました。
自分の目ではソースを読み切るのが難しく、LLMと一緒に作業してみました。いい経験になりました！