How to implement multi-level routing in Angular?

I'm working on a little project of mine in order to learn something more about Angular, but I really cannot figure out how to implement a multi-leveled routing.

I've read the documentation about the new release of the Router Component and also some other topics on StackOverlfow (first, second, third), but I cannot find a solution to my problem.

Let's consider the following app structure, without considering the Test and Test2 blocks.

And let's consider the components of my app as following:

main.ts

import { bootstrap }    from '@angular/platform-browser-dynamic';

import { MyAppComponent } from './my-app/my-app.component';
import { APP_ROUTER_PROVIDERS } from './my-app/my-app.routes';

bootstrap(MyAppComponent, [ APP_ROUTER_PROVIDERS ])
    .catch(err => console.error(err));

my-app.component.ts

import { Component } from '@angular/core';
import { ROUTER_DIRECTIVES } from '@angular/router';

@Component({
    selector: 'my-app',
    template: '<router-outlet></router-outlet>',
    directives: [ROUTER_DIRECTIVES],
})

export class MyAppComponent { }

my-app.routes.ts

import { provideRouter, RouterConfig } from '@angular/router';

import { HomeComponent } from './home/home.component';
import { LoginComponent } from './login/login.component';
import { SignUpComponent } from './sign-up/sign-up.component';

import { AdminRoutes} from './admin/admin.routes';

export const routes: RouterConfig = [
  { path: '', component: HomeComponent },
  { path: 'login', component: LoginComponent },
  { path: 'sign-up', component: SignUpComponent },
  ...AdminRoutes,
];

export const APP_ROUTER_PROVIDERS = [
    provideRouter(routes)
];

admin.component.ts

import { Component } from '@angular/core';
import { ROUTER_DIRECTIVES } from '@angular/router';

@Component({
    selector: 'admin',
    template: 'Hello I am ADMIN <br> <router-outlet></router-outlet>',
    directives: [ROUTER_DIRECTIVES]
})

export class AdminComponent { }

admin.routes.ts

import { RouterConfig } from '@angular/router';

import { AdminComponent } from './admin.component';
import { MainPageComponent } from './main-page/main-page.component';
import { SettingsComponent } from './settings/settings.component';

export const AdminRoutes: RouterConfig = [
  {
      path: 'admin',
      component: AdminComponent,
      children: [
        { path: 'main-page', component: MainPageComponent },
        { path: 'settings', component: SettingsComponent },
      ]
  }
];

main-page.component.ts

import { Component } from '@angular/core';

@Component({
    selector: 'main-page',
    template: 'Hello I am MAIN PAGE!!!'
})

export class MainPageComponent { }

settings.component.ts

import { Component } from '@angular/core';

@Component({
    selector: 'settings',
    template: 'Hello I am SETTINGS!!!'
})

export class SettingsComponent { }

When I try to use such a configuration nothing works anymore and the browser's console is full of errors.

This happens only when I add the children[...] in the admin.routes.ts, and I think the problems come in there.

1. Could you give me any hint on how I can implemented properly, please?
2. Am I missing anything?
3. Is there a better way?

Thank you in advance for your help and I hope what I've written in the post it's helpful to understand my issue!