Rome’s city population swelled from an influx of non-citizens — favor-seekers, merchants, professionals, entertainers — including many slaves and ex-slaves:
Somewhat surprisingly, being a slave in an important Roman family was a path to upward mobility, since slaves did most of the household and administrative work (being a slave in agriculture or mining was a different story) and many of them were eventually freed as an incentive for loyal service.
Since old Roman conservatives looked down on business, ex-slaves became part of the growing capitalist class. Most important of all was a class of capitalists who leased the state’s public land, since they had the capital to achieve economies of scale in working large plantations, mines, timber, and importing the food supply to feed the population of Rome. It was a minimalist state in most respects. Rome owned vast properties but had few public officials, and they were appointed to very short terms. Hence most public enterprises were leased out; capitalists undertook to collect taxes, advancing cash for state needs and squeezing what they could out of subject peoples.
The New Testament gives us a glimpse of these Roman citizens out in the provinces: Jesus offended local ethnic loyalties by converting tax collectors; and Paul himself was a Roman citizen. Since the most important state organization was the army, the biggest state-related business was supplying it with weapons, armor, food, ships, and harbors. Rome thus developed its “military-industrial complex”, similar to the US since late 20th century in outsourcing as much as possible to private contractors.