To optimize performance in Angular applications, especially when dealing with large lists and immutable data, you should use the ChangeDetectionStrategy.OnPush strategy. This strategy minimizes change detection cycles by only checking components when their inputs change. It can significantly improve performance. The other options are not valid strategies for optimizing change detection.

The primary purpose of the RouterOutlet directive in Angular is to act as a placeholder for route content. It is used in the template to determine where the routed components should be displayed within the layout. This is essential for rendering the correct component when navigating between routes. The other options do not accurately describe the role of RouterOutlet in Angular.

When making an HTTP request using HttpClient, an Observable object is returned. Observables are used to handle asynchronous data streams and provide features like error handling and transformation, making them well-suited for managing HTTP responses.

In Angular, to define routes for an application, you typically create an array of Route objects. These objects define the mapping between routes and components, specifying which component should be displayed when a particular route is navigated to. The RouterModule is indeed used to set up the routing configuration in an Angular application, but it's not an array. Components and modules are essential parts of Angular, but they are not used directly to define routes.

It is recommended to use immutable data structures with the "OnPush" change detection strategy because immutable data structures help avoid side effects. In Angular, when you change an input property's value, it triggers change detection. Using mutable objects can lead to unintended changes and unexpected behavior. Immutable data structures ensure that once a value is set, it remains unchanged, making it easier to reason about component behavior and performance optimization.

These lifecycle hooks, ngAfterViewInit and ngAfterViewChecked, are related to a component's view rendering process in Angular. ngAfterViewInit is called after the view and its child views (if any) have been initialized, and ngAfterViewChecked is called after every check of the component's view and child views. These hooks allow developers to perform actions after the view has been created or updated.

In Angular, when creating custom form controls, the method used to write a new value to the view is setValue(). This method allows you to set the value of a FormControl and update the view accordingly. It's a critical method for working with form controls in Angular.

To accommodate the client's requirement of deep linking to specific parts of your Angular application, you should use Angular's RouterModule and configure the application for hash-based routing. This approach allows for deep linking by using URL fragments. The other options are not suitable for achieving deep linking in an Angular application.

You would consider using ChangeDetectionStrategy.OnPush in scenarios where the component's updates depend on changes in its parent component. This strategy is beneficial when you want to optimize performance by reducing unnecessary change detection cycles. It's not typically used when a component needs to check for changes on every interaction or when it's rarely used.

The primary difference is that switchMap cancels the previous inner observable when a new one arrives. In contrast, mergeMap concurrently processes all inner observables it receives. This behavior makes switchMap useful for scenarios where you want to ensure only the latest inner observable result is considered, while mergeMap retains all results from concurrently executing inner observables.