What Sigma lens should I buy for my Sony camera to take pictures of people and buildings while walking around? I just got an a6400 and honestly I have no idea where to start with all the different numbers on the boxes. My budget is around $500 because I spent most of my money on the camera body itself.

I am going on a trip to Chicago in three weeks and I really want to capture the vibe of the city but I dont want a huge lens that makes people stare at me. I keep seeing people talk about the 30mm or maybe a 16mm? I dont even know what those numbers change. Is there one that is just a good all-rounder for a beginner who doesnt want to keep switching lenses?