To call a backend API from an Angular application, you can use the Angular HTTP Client. Here is an example of how to make an HTTP GET request to an API using the Angular HTTP Client:

import {HttpClient} from '@angular/common/http';

@Injectable({
    providedIn: 'root'
})
export class ApiService {
    constructor(private http: HttpClient) {
    }

    getData() {
        return this.http.get('https://api.example.com/endpoint');
    }
}

In this example, the ApiService is an injectable service that makes an HTTP GET request to the specified API endpoint using the http.get() method. The API endpoint can be any URL that you want to call, such as a server-side API or a third-party API.

To use the ApiService in a component, you can inject it into the component’s constructor and call the getData() method:

import {ApiService} from './api.service';

@Component({
    ...
})
export class MyComponent {
    constructor(private apiService: ApiService) {
    }

    ngOnInit() {
        this.apiService.getData().subscribe(data => {
            console.log(data);
            // Process the data received from the API
        });
    }
}

The getData() method returns an observable that emits the data received from the API when the request is successful. The subscribe() method is used to subscribe to the observable and process the data.

This is just a basic example of how to call a backend API using the Angular HTTP Client. You can find more information about using the HTTP Client in the Angular documentation.