Angular API folder (how to mount this project?)

Question

I took this project from a tutorial. The porblema is that it does not clarify where I have to put the folder "api" inside the Angular CLI project is giving error and the sql connection does not work. Where do I put this folder to stop giving this error?

Failed to load Resource: net::ERR_CONNECTION_REFUSED

service

    import { Injectable } from '@angular/core';
import { HttpClient, HttpErrorResponse, HttpParams } from '@angular/common/http';

import { Observable, throwError } from 'rxjs';
import { map, catchError } from 'rxjs/operators';

import { Car } from './car';

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

  baseUrl = 'http://localhost/api';

  cars: Car[];

constructor(private http: HttpClient) { }

  getAll(): Observable<Car[]> {
    return this.http.get(`${this.baseUrl}/list`).pipe(
      map((res) => {
        this.cars = res['data'];
        return this.cars;
    }),
    catchError(this.handleError));
  }
}

api/connect.php

 <?php

// db credentials
define('DB_HOST', 'localhost');
define('DB_USER', 'root');
define('DB_PASS', '');
define('DB_NAME', 'db');

// Connect with the database.
function connect()
{
  $connect = mysqli_connect(DB_HOST ,DB_USER ,DB_PASS ,DB_NAME);

  if (mysqli_connect_errno($connect)) {
    die("Failed to connect:" . mysqli_connect_error());
  }

  mysqli_set_charset($connect, "utf8");

  return $connect;
}

$con = connect();