The city of Rome was a huge urban center, and of course was not only populated by the affluent. What would the average person, who obviously could not farm on the city streets, do for a living?
I think this thread should serve you well
The short version is that the idea of a steady job or an "occupation" is not something that applies to the vast majority of the working population of the city. Most people in the Roman world by far were farmers of various means, but within the city only a relatively few could consider themselves part of the plebs media, the shopkeepers and skilled workers who could claim to have steady employment, generally in their own shops. By far the majority of the urban population was made up of itinerant day workers, who lived in abject poverty and chased whatever work was available. The most frequent type of work was in construction, particularly of monumental works, and as labor for the grain merchants--Vespasian refused to employ a labor-saving device of some kind because it would take away what little work many of the urban plebs could rely on.