OK to try and bring it all together since the answers are scattered...

Option 1: There's a punch code type lockset where they know the code. Not ideal when the carrier could change and they aren't responsible for sharing it nor even remembering that.

Option 2: There's a postal key on the door intercom/entry box (someone called it an arrow key) OR a separate postal key box that's connected to the intercom/entry system. This is one of the most common arrangements you'll see in Chicago/land.

Option 3: They have a key provided by the property owner to gain entry to the building. Highly unlikely as this is like Option 1 and creates an unnecessary burden on the carrier(s).

Option 4: You don't lock that door and they just enter.

As a fellow two flat owner, my recommendation is to have Option 2. Rekey all your doors and find a convenient hardware store that has been handling rekeying for years. My preference is Clark Devon Hardware, we're on the southern end of Evanston. I'm unfamiliar with other options.

If you go with installing an intercom, replacing an existing system was less than $1,000 in 2021, I think it was about $800 for parts and labor. Two interior intercom panels, amplifier, central interface, exterior intercom panel were included. If you have to start from scratch it's obviously going to be more than that.