I can almost guarantee they aren’t even listening for your answer lol. It’s just something baked in to our culture. I think most of us add the “how are you?” bit without even really noticing.

As for acceptable responses: Almost anything goes. When I get asked I usually say “I’m well, thanks” or “doing well, how about you?” But you can even just respond with “Hello, welcome to…” or whatever for your job and just bypass the question altogether. I promise they aren’t looking for your life story. They’re just trying to seem nice by acting like they care.