Apple ID Sign In: AppleAuth First Step

AppleAuth.java

private void appleAuth(String authorizationCodeSentByApp, String keyIdentifierSentByApp) throws Exception {
  var token = generateJWT();

  //Http Requests Client
  var tokenResponse = httpClient.getTokenResponse("CLIENT_ID", token, "authorization_code", authorizationCodeSentByApp);

  var idToken = tokenResponse.getIdToken();
  
  // Next Steps
}

Where is httpClient defined?
Exactly