I am more read on French and UK prostitution, but here are some things to consider:

There has never really been a 'sex industry'. Prostitution has always, and will always be made up of similar, but barely connected industries.

Street prostitution was prevalent in all of Europe, and was mentioned from medieval times onwards, especially in church records, as they were especially concerned with the morality of the poor, who are the most likely to be involved in 'survival sex'. Many cities attempted to restrict street prostitution to certain districts (or the workers clustered where the business was best) and these areas live on in colorful street names.

The next tier were bar girls, who frequented bars/taverns etc and worked much like saloon prostitutes that you see in Westerns, or the current Asian bar girl trade. You pay a 'fine' to the house, and then you rent a room upstairs and 'tip' the girl. Some establishments were fronts for brothels, but others simply tolerated sex workers for the entertainment of the patrons. These houses catered to the working poor, and anybody with more specialized tastes requiring 'discretion'.

Then you have the brothels, which have been commonplace in cities for most of recorded history. Brothels are also class segregated. There were always many brothels catering for the working poor, sailors and soldiers etc. Stories of these places frequently turn up in legal records etc, as periodic campaigns were waged to close or regulate various establishments.

Cities such as Paris and Venice were famous for 'Salon' prostitutes, or high class brothel workers. These brothels were often famously opulent. Workers were famed for their beauty and/or charm. Many would cross over to becoming mistresses of rich merchants etc. Many moved between the brothels and the performing arts with some fluidity. Some would go on to enduring fame and competed directly with courtesans.

Workers were segregated by beauty standard and social class, although it was possible for particularly attractive and charming working class girls to make the jump to working in salons. As a whole, sex workers maintained just as rigid a social structure as general society. 'Unionisation' of sex workers was unlikely. A salon prostitute would not necessarily be moved by the crackdown on street prostitution, and may have been insulated from the pressures that other sex workers face. Some may have had political influence, or were at least able to patronise local fashion houses, artists, the theatre etc, using their own patrons money to amuse themselves. As it stands now, 1/3 of the money spent on luxury goods in China is by, or for, mistresses, so such a culture can have a large influence on the economy and the arts.

'Mistresses' can be somebody of a similar standing and social class such as a courtesan, but they can also be a young and desirable woman that is kept on retainer. The chinese differentiate, between the two, calling 'high class' mistresses xiao san (or 'third party') and the girls on retainer are called ernai. Ernai tend to be of low social standing and education, and are often met at brothels or hostess bars. Rich men there will have one xiao san, who is often a career woman herself, and is considered 'polite' enough to attend events etc, much like a courtesan. In some of the recent court cases officials have been revealed to be keeping as many as a dozen ernai on retainer. Historical western mistress culture is usually only discussed in the contect of royal mistresses, but figures such as Samuel Pepys recorded their dalliances with household staff or servant girls, so it is a not unreasonable assumption that men of even modest wealth kept women if possible.