Why jest mock not working if it isn't declared in 'it' scope

I created a simple typescript project with create-react-app..
And I try to create unit test with a jest's mock.
the class, I want to mock :

export class RestUserAPI {
  public constructor() {}
    getName(): string {
      return "MyUser";
    }
  }
}

the class I want to test

import {RestUserAPI} from "./RestUserAPI";

export class UserService {
  private userAPI: RestUserAPI;
  public constructor() {
    this.userAPI = new RestUserAPI();
  }

  getName(): string {
    return this.userAPI.getName();
  }
}

And my test which is not working

import { UserService } from './UserService';
import {RestUserAPI} from "./RestUserAPI";

const mockedUserName = 'mocked user name'

jest.mock('./RestUserAPI');
const RestUserAPIMock = RestUserAPI as jest.MockedClass<typeof RestUserAPI>;
RestUserAPIMock.mockImplementation(() => {
   return {
     getName: jest.fn().mockReturnValue(mockedUserName),
   };
});

describe("User Service", () => {
  let instance: UserService;
  beforeAll(() => {
    RestUserAPIMock.mockClear()
  });

  it("should get user name", () => {
    instance = new UserService();
    expect(instance.getName()).toBe(mockedUserName);
    expect(RestUserAPIMock).toHaveBeenCalledTimes(1);
  });
});

this code work, butt this is not what I want

import { UserService } from './UserService';
import {RestUserAPI} from "./RestUserAPI";

const mockedUserName = 'mocked user name'

jest.mock('./RestUserAPI');
const RestUserAPIMock = RestUserAPI as jest.MockedClass<typeof RestUserAPI>;

describe("User Service", () => {
  let instance: UserService;
  beforeAll(() => {
    RestUserAPIMock.mockClear()
  });

  it("should get user name", () => {
    RestUserAPIMock.mockImplementation(() => {
      return {
        getName: jest.fn().mockReturnValue(mockedUserName),
      };
    });

    instance = new UserService();
    expect(instance.getName()).toBe(mockedUserName);
    expect(RestUserAPIMock).toHaveBeenCalledTimes(1);
  });
});

What is my mistake?
How can I create a mock implementation for all my 'it' blocks?