fill your data in behaviourSubject then subscribe to it, now your behaviourSubject method will be called whenever you data get changed, here is an example:

I declare my behaviourSubject like these:

  private _resetForm$ = new BehaviorSubject<boolean>(false);


      public set setResetForm(resetStatus: boolean) {
        this._resetForm$.next(resetStatus);
      }
    
      public get resetForm(): Observable<boolean> {
        return this._resetForm$.asObservable();
      }

then I change it here:

  this.setResetForm = true;

now i will get event from here because setResetForm have changed so i will get my event from it:

  ngOnInit(): void {
    this.resetForm.subscribe((res: boolean) => {
      if (res) {
        console.log(res);
      }
    });
  }

good luck!

Based on your short explanation here is a general solution:

wrap your items assignment in a method and call this method when you set the itemCategories value.

async ngOnInit(){
   this.setItems();
}

setItems(){
   this.items = await this.service.getItems();
}

setItemCategories(value){
   this.itemCategories = value;
   this.setItems();
}

You can use the EventEmitter and @Output() to achieve your goal.
Dropdown menu component:

import { Component,  Output, EventEmitter, NgModule } from '@angular/core';
@Component({
  selector: 'app-dropdown',
  template: `
    <div>
      <select [(ngModel)]="selected" (ngModelChange)="onChange($event)">
        <option *ngFor="let category of categories" [value]="category">{{category}}</option>
      </select>
    </div>`
})
export class DropdownComponent {
  @Output() selectedCategory: EventEmitter<string> = new EventEmitter<string>();
  categories = ["All categories", "A", "B", "C"]; //your categories list
  selected: string = "All categories"; //default value 
  constructor() {
  }

  onChange(newvalue): void {
    this.selectedCategory.emit(newvalue);
  }

}  

Here is the other component that will receive the selected value whenever it changes from the dropdown component

import {Component, NgModule} from '@angular/core'
@Component({
  selector: 'app-main',
  template: `
    <div>
      <app-dropdown (selectedCategory)='onChange($event)'></app-dropdown>
    </div>
  `,
})
export class MainComponent {
  selected: string;
  constructor() {
  }

  onChange(category):void {
    this.selected = category;
    //get your items list based on the selected category
  }
}