MRE for https://stackoverflow.com/q/61777103/3001761

demo.service.spec.ts

import { TestBed } from '@angular/core/testing';
import { HttpClientTestingModule, HttpTestingController } from '@angular/common/http/testing';

import { DemoService } from './demo.service';

fdescribe('DemoService', () => {
  let service: DemoService;
  let httpMock: HttpTestingController;

  beforeEach(() => {
    TestBed.configureTestingModule({
      imports: [
        HttpClientTestingModule
      ],
    });
    service = TestBed.inject(DemoService);
    httpMock = TestBed.inject(HttpTestingController);
  });

  afterEach(() => {
    httpMock.verify();
  });

  it('should be created', () => {
    service.makeRequest('hello').subscribe((result) => {
      expect(result).toEqual({ foo: 'bar' });
    });

    httpMock.expectOne('domain.ext/hello').flush({ foo: 'bar' });
  });
});

demo.service.ts

import { Injectable } from '@angular/core';
import { HttpClient, HttpHeaders } from '@angular/common/http';
import { of, Observable, forkJoin } from 'rxjs';
import { flatMap } from 'rxjs/operators';

@Injectable({
  providedIn: 'root'
})
export class DemoService {

  constructor(private http: HttpClient) { }

  makeRequest(path: string): Observable<any> {
    return forkJoin([
      this.createUri(path),
      this.createHeaders()
    ]).pipe(
      flatMap(([url, headers]) => this.http.get(url, { headers }))
    );
  }

  private createHeaders(): Observable<HttpHeaders> {
    return of(new HttpHeaders());
  }

  private createUri(path: string): Observable<string> {
    return of(`domain.ext/${path}`);
  }
}