They were built in the early 3rd Century and remained in use until the 6th Century when the Goths wrecked the aqueduct that supplied water. Barbarians. At one time, they had marble walls and all sorts of statues and other works of art. Over time, people stole the art, marble and anything else valuable and reasonably portable. Much of the art can still be seen in Italian museums. We saw some when we were down in Naples.
You can see the size in the photos. Floors originally were covered in mosaics. You can see volunteers restoring some of the mosaics. It takes a lot of time and attention to detail.
The Romans has a more communal civic culture than we do. They tended not to have things like baths or even kitchens in their homes, so they took care of these needs in public. When I was growing up in Milwaukee we had public natatoriums, local version of baths. We used to go to swim in the pool, but some people came to use the showers, since their houses did not have them.