@Input() value is always undefined

It will be initialized in ngOnInit, not the constructor. (Please also review the Angular Life Cycle Hooks documentation.)

ngOnInit() {
  console.log('userId is:',this.userId);
}

Also if you want to pass a literal like a string and use brackets [] you have to pass it as a string by enclosing the value with single ticks.

<user-photos [userId]="'TestingInput'"></user-photos>

The reason for that is the containing expression is evaluated in the context of the containing component so without it it will try to retrieve and pass a property/field named TestingInput which will be undefined (unless you also happen have a field by that name).

In my case i had to use *ngIf = "isLoaded" on the parent's template first.

On Parent Component

    <div *ngIf = "isLoaded">
       <app-child-component [dataToChild]="dataFromParent"> </app-child-component>
    </div>

On Child Component

      @Input() dataToChild: any;
        constructor() { }
        ngOnInit(): void {
             console.log(this.dataToChild);
         }