[TypeScript][Angular]メソッド(関数)の引数の中に関数？助けてください。

下記コードのupdateメソッドの仮引数(fn: (state: State) => State)とはどういう意味でしょうか？
このメソッドが呼び出されたとき引数には何が入ってくるのでしょうか？

コードを読んでいますが動きが読めません。助けてください

store.service.ts

tsx
1import { Injectable } from '@angular/core';
2
3import { BehaviorSubject, queueScheduler } from 'rxjs';
4import { map, distinctUntilChanged } from 'rxjs/operators';
5import { State, initialState } from './state';
6
7@Injectable({ providedIn: 'root' })
8export class Store {
9
10  private _state$ = new BehaviorSubject<State>(initialState);
11
12  update(fn: (state: State) => State) {
13    const current = this._state$.value;
14
15    queueScheduler.schedule(() => {
16      this._state$.next(fn(current));
17    });
18  }
19
20  select<T>(selector: (state: State) => T) {
21    return this._state$
22    .pipe(
23      map(selector),
24      distinctUntilChanged(),
25    );
26  }
27}

user-detail.usecase.ts

tsx
1import { Injectable } from '@angular/core';
2import { Store } from './store.service';
3import { UserApiService } from './user-api.service';
4
5@Injectable({ providedIn: 'root' })
6export class UserDetailUsecase {
7  get user$() {
8    return this.store.select(state => state.userDetail.user);
9  }
10
11  constructor(private userApi: UserApiService, private store: Store) {}
12
13  async fetchUser(userId: string) {
14    this.store.update(state => ({
15      ...state,
16      userDetail: {
17        ...state.userDetail,
18        user: null
19      }
20    }));
21
22    const user = await this.userApi.getUserById(userId);
23
24    this.store.update(state => ({
25      ...state,
26      userDetail: {
27        ...state.userDetail,
28        user
29      }
30    }));
31  }
32}

tsx
1import { User } from './user';
2
3export interface UserListFilter {
4  nameFilter: string;
5}
6
7export interface State {
8  userList: {
9    items: User[];
10    filter: UserListFilter
11  },
12  
13  userDetail: {
14    user: User | null;
15  }
16}
17
18export const initialState = {
19  userList: {
20    items: [],
21    filter: {
22      nameFilter: '',
23    }
24  },
25  userDetail: {
26    user: null
27  }
28};