What is the APP_INITIALIZER token and how is it used to load configuration at startup?

import { Injectable, inject } from '@angular/core';
import { HttpClient } from '@angular/common/http';
import { tap } from 'rxjs';

@Injectable({ providedIn: 'root' })
export class ConfigService {
  private http = inject(HttpClient);
  private settings: any = null;

  get apiBaseUrl() { return this.settings?.apiUrl; }

  loadSettings(): Promise<void> {
    return new Promise((resolve, reject) => {
      this.http.get('/assets/config.json').pipe(
        tap(cfg => this.settings = cfg)
      ).subscribe({
        next: () => resolve(),
        error: (err) => {
          console.error('Config failed to load', err);
          resolve();
        }
      });
    });
  }
}

// app.config.ts
import { ApplicationConfig, APP_INITIALIZER } from '@angular/core';
import { provideHttpClient } from '@angular/common/http';

export const appConfig: ApplicationConfig = {
  providers: [
    provideHttpClient(),
    {
      provide: APP_INITIALIZER,
      useFactory: (configSvc: ConfigService) => () => configSvc.loadSettings(),
      deps: [ConfigService],
      multi: true
    }
  ]
};