ngDoBootstrap vs NgModule.bootst

When we bootstrap an angular application, which component should angular runtime start? or which component is root component. Here is the way:

Option 1: NgModule.bootstrap attribute

This is the most used option, @angular/cli default generated, in angular guides and examples:

@NgModule({
  declarations: [
    AppComponent,
  ],
  imports: [
    BrowserModule,
  ],
  providers: [  ],
  bootstrap: [AppComponent],
})
export class AppModule implements DoBootstrap {
}

Option 2: implement DoBootstrap interface

@NgModule({
  ...
  entryComponents: [AppComponent],
})
export class AppModule implements DoBootstrap {
  ngDoBootstrap(appRef: ApplicationRef): void {
    appRef.bootstrap(AppComponent);
  }
}

Maybe angular compiler generate the appRef.bootstrap() call somewhere.