Angular: Lazy loading modules with services

providedIn: 'root' is the easiest and most efficient way to provide services since Angular 6:

1. The service will be available application wide as a singleton with no need to add it to a module’s providers array (like Angular <= 5).
2. If the service is only used within a lazy loaded module it will be lazy loaded with that module
3. If it is never used it will not be contained in the build (tree shaked).

For further informations consider reading the documentation and NgModule FAQs

Btw:

1. If you don’t want an application-wide singleton, use the provider’s array of a component instead.
2. If you want to limit the scope so no other developer will ever use your service outside of a particular module, use the provider’s array of NgModule instead.*

*UPDATE

‘use the provider’s array of NgModule instead’ means to use the providers array of the lazy loaded module, eg:

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

import { UserService } from './user.service';

@NgModule({
  providers: [UserService],
})
export class UserModule {
}

OR to actually name the module in the injectable decorator:

import { Injectable } from '@angular/core';
import { UserModule } from './user.module';

@Injectable({
  providedIn: UserModule,
})
export class UserService {
}

Quote from the docs:

When the router creates a component within the lazy-loaded context,
Angular prefers service instances created from these providers to the
service instances of the application root injector.

Doc ref: https://angular.io/guide/providers#providedin-and-ngmodules