Can I get the code of a TestFlight app? I have hired a developer to create an iOS app. He was working on it and showing me the progress on TestFlight but now he is not responding, don't know what's going on.

Now can I get the code from a TestFlight so that I can hire a new developer who can start from where the previous guy left?
No - you might retain a copy of the application by transferring it to iTunes in case you take legal action to arbitrate or ask the courts to assist you with your contract with your developer.

Apple doesn't receive code from developers when an app is submitted (test flight submissions work the same as app store submissions) - just the compiled program.