How to use the @loopback/core.Application function in @loopback/core

it('honors expressSettings', () => {
        const app = new Application();
        const server = new TestRestServer(app, {
          expressSettings: {
            'x-powered-by': false,
            env: 'production',
          },
        });
        const expressApp = server.expressApp;
        expect(expressApp.get('x-powered-by')).to.equal(false);
        expect(expressApp.get('env')).to.equal('production');
        // `extended` is the default setting by Express
        expect(expressApp.get('query parser')).to.equal('extended');
        expect(expressApp.get('not set')).to.equal(undefined);
      });

it('honors "x-forwarded-host" headers', async () => {
    const app = new Application();
    app.component(RestComponent);
    const server = await app.getServer(RestServer);

    const response = await createClientForHandler(server.requestHandler)
      .get('/explorer')
      .set('x-forwarded-proto', 'http')
      .set('x-forwarded-host', 'example.com:8080,my.example.com:9080');
    await server.get(RestBindings.PORT);
    const expectedUrl = new RegExp(
      [
        'http://explorer.loopback.io',
        '\\?url=http://example.com:8080/openapi.json',
      ].join(''),
    );
    expect(response.get('Location')).match(expectedUrl);
  });

async function givenAServer(
    options: {rest: RestServerConfig} = {rest: {port: 0}},
  ) {
    options.rest = givenHttpServerConfig(options.rest);
    const app = new Application(options);
    app.component(RestComponent);
    return app.getServer(RestServer);
  }

function givenRequestContext() {
    app = new Application();
    reqCtx = new Context(app);
    reqCtx
      .bind(SecurityBindings.USER)
      .to({[securityId]: 'user-01', name: 'user-01'});
    controller = new OrderController();
  }

function givenApplication() {
    return new Application();
  }
});

function givenApplication() {
    return new Application();
  }
});

@inject(CoreBindings.APPLICATION_CONFIG)
            config?: RestComponentConfig,
          ) {
            super(application, config);
          }
        }

        class CustomLogger implements Provider {
          value() {
            return (err: Error, statusCode: number, request: Request) => {
              lastLog = `${request.url} ${statusCode} ${err.message}`;
            };
          }
        }

        const app = new Application();
        app.component(CustomRestComponent);
        const server = await app.getServer(RestServer);
        const logError = await server.get(SequenceActions.LOG_ERROR);
        const expressContext = stubExpressContext({url: '/'});
        logError(new Error('test-error'), 400, expressContext.request);

        expect(lastLog).to.equal('/ 400 test-error');
      });
    });

function givenApplicationAndAuthorizer() {
    app = new Application();
    app.component(AuthorizationComponent);
    app.bind('casbin.enforcer').toDynamicValue(createEnforcer);
    app
      .bind('authorizationProviders.casbin-provider')
      .toProvider(CasbinAuthorizationProvider)
      .tag(AuthorizationTags.AUTHORIZER);
  }