In RxJS, the forkJoin operator is used when you need to combine multiple Observables and wait for all of them to complete before emitting a result. It will emit an array of the last values from each Observable when they have all completed.

To implement an undo feature in NgRx, you would leverage "Time-travel debugging." This feature allows you to step backward and forward through the state changes, effectively enabling undo and redo functionality in your application.

In Angular's Reactive Forms, you can implement conditional form field validation by using a custom validator function. This function can check the value of one field and set the validation rules for another field accordingly. Template-driven forms are a different approach and do not provide as much flexibility in handling complex validation logic. Creating a new component for each form field is not the recommended approach for conditional validation, and Angular Material primarily deals with UI components, not the underlying validation logic.

The '*' syntax in directives like *ngIf and *ngFor is a syntactic sugar for using Template expressions. In Angular, Template expressions are embedded within double curly braces {{ }} and allow you to evaluate expressions, access properties, and perform other operations within the template. This syntax simplifies and enhances the readability of Angular templates.

To create a directive that listens for host events and reacts accordingly, you should use the @HostListener decorator. This decorator allows you to define event listeners directly within the directive class, making it easy to respond to events on the host element where the directive is applied.

To avoid unnecessary change detection cycles for components with data that doesn't change frequently, you can set their change detection strategy to ChangeDetectionStrategy.OnPush. This strategy makes the component's change detection dependent on its input properties, resulting in better performance. The Default strategy doesn't optimize for this scenario. Lazy and Smart are not valid change detection strategies in Angular.

In Angular, the *ngIf directive is used to conditionally display an element in the DOM. It evaluates an expression and only renders the associated element if the expression is true. This is commonly used for showing or hiding elements based on certain conditions. *ngFor is used for looping over arrays and rendering lists, not for conditional rendering. *ngSwitch is used for switching between multiple views based on a condition, and *ngClass is used for dynamically adding or removing CSS classes.

Immutable data structures, such as those provided by libraries like Immutable.js or using TypeScript's 'readonly' modifier, enhance performance in Angular applications by reducing the need for change detection. This is because changes to immutable data structures result in entirely new objects, making it easier for Angular to identify changes and update the view efficiently.

In Angular's Reactive Forms, the FormBuilder is a service that provides convenient methods to construct form controls. It simplifies the process of creating and managing form controls and their corresponding form groups. Using FormBuilder, developers can easily define form structures with various input types and validation rules.

In Angular's hierarchical dependency injection system, the root injector is the first injector that is checked when trying to resolve a service dependency. The root injector provides a global scope for services, and if the service is not found there, Angular will progressively look for it in other injectors further down the hierarchy. Understanding this hierarchy is essential for managing service dependencies in Angular applications.