app.component.ts

import { Component, OnInit } from '@angular/core';
import { SingletonService } from './singleton.service';

@Component({
  selector: 'app-root',
  templateUrl: './app.component.html',
  styleUrls: ['./app.component.scss']
})
export class AppComponent implements OnInit {
  constructor(
    private singletonService: SingletonService
  ) { }

  ngOnInit(): void {
    this.singletonService.someMethod();
  }
}

app.module.ts

import { BrowserModule } from '@angular/platform-browser';
import { NgModule } from '@angular/core';

import { AppComponent } from './app.component';
import { ModuleAModule } from './moduleA/moduleA.module';
import { SingletonService } from './singleton.service';

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

moduleA.component.ts

import { Component, OnInit } from '@angular/core';
import { SingletonService } from '../singleton.service';

@Component({
  selector: 'app-module-a',
  templateUrl: './moduleA.component.html',
  styleUrls: ['./moduleA.component.scss']
})
export class ModuleAComponent implements OnInit {
  constructor(
    private singletonService: SingletonService
  ) { }

  ngOnInit(): void {
    this.singletonService.someMethod();
  }
}

moduleA.module.ts

import { NgModule } from '@angular/core';
import { SingletonService } from '../singleton.service';

import { ModuleAComponent } from './moduleA.component';

@NgModule({
  declarations: [
    ModuleAComponent
  ],
  exports: [ModuleAComponent],
  providers: [SingletonService],
})
export class ModuleAModule { }