in cities its likely to be smells from places of businesses, homes, and people, as well as things like gas and diesel.

Unsure how relevant burning garbage would be, depends on how clean they burn and how well they pull out particulates i guess. Worth doing some reading on later perhaps.

In places like NYC its not uncommon to get sewer gas venting in the streets, you also have rats, garbage, and some amount of homeless people in the more populated areas obviously.

I would also probably be inclined to argue that a significant part of the "city smell" is also likely to be the lack of country smell, there's all kinds of shit out there, sometimes literally, most of the time it's just shit like foliage. Wet rotting organic material really carries through the air.

quick edit: as for diesel, engines burn the fuel a lot more completely today, a lot of modern diesels also burn DEF (diesel exhaust fluid) to capture some of the more aggressive particulates that cause pollution and other issues. If an older 60's 70's car drives right be you, you get a face full of gas exhaust and you can smell it. Modern car exhausts mostly smell like death because they burn basically everything they can. There's also a lot less particulates so it doesnt spread as much.

and obviously, the formulation of gas has changed over time, same for diesel probably.