No one here will be able to tell you anything personally, because flophouses pretty much don’t exist anymore, now that there are fire laws and occupancy laws.
Basically, someone who owned a big building in a poor part of town would cram it full of narrow bunks, and for very little money, homeless people could rent a bed for a night on a first-come, first-served basis.
A few women might sweep up during the day in exchange for their bed, and once a month or so, the sheets and blankets would get washed and hung to dry. The owner (or manager) might solicit volunteers among the population to help with that chore in exchange for a free bed for a night.
A bed might cost five cents. In the morning, there might be coffee for a penny for those who had stayed the night. In general, they didn’t really offer anything in the way of breakfast, though.
Imagine if a homeless shelter were unregulated, so they crammed as many bunks in as they physically could, and had no cleanliness standards, and charged something very minimal to stay there, with none of the other charitable services homeless shelters have, like vouchers to take the bus to a soup kitchen, or help getting appointments with social services, or rehab programs. No showers. They just rented you a bed, and that was it. Maybe also a bedpan under the bed, just to keep people from peeing directly on the floor.
They were pretty nasty places, but better than sleeping in they alley when it was cold or raining.
If you read a lot, you read about them a lot.
YMCAs were established mainly as a cleaner, safer alternative to flop houses, but you had to be sober, and you had to listen to a sermon or two. In big cities, YMCAs still have (tiny) rooms to rent on a daily basis to people who are essentially homeless. But in most of the US, “Ys” are just a type of gym franchise.