Is my battery dead or my alternator or both or neither?

Hello everyone,

This is a follow up to this post:

https://www.reddit.com/r/askcarguys/comments/1rbqfdk/comment/o6tz4ax/

I bought a battery tester and the tester showed "Empty". When I tried to start the car, I forgot to remove the tester. But now the tester shows "Dead".

When I put the key in the ignition and turn it, lights come on and the radio also works. But when I try to start it, it just does a clicking noise for a second before everything shuts off. Then, when I turn the key, nothing happens. So I then tried jump starting it but all it did was that nothing shut off and the clicking noise kept going. I didn't let it go to long so I didn't break my starter.

So, I'm guessing a dead battery. But how can I know if it's actually the alternator that is bad? Or did the battery tester kill my battery? Can it be something else?

I had both the alternator and the battery changed in June but in a place that is known for cutting corners so I'm a little worried that's what happened.

Thank you!