Japan is one of those countries that continue to have public baths. Open to most of the public, they usually have a sign banning people with tattoos, tattoo stickers, heavily drunk people, and pets. But once you get past the entrance and into the changing room, on the men's side at least, you can typically spot the yakuza with a large tattoo on his back. It turns out that enforcing the rule doesn't happen as often as you'd think.
Of the public baths in Japan, onsen is more for tourists, and it is usually associated with a ryokan, or an inn. It may be packaged with meals and a place to sleep. Sentō on the other hand is more for the locals. Just the bathing experience. Public bathing is a rather old concept, but for some set of reasons, public baths still exist in Japan, although steadily losing customers, especially among the younger generations. In stark contrast, modern day US has virtually none.
In anime, people go to sento when apartment boilers break, or they live in cheaper neighborhoods without private baths, and the anime series has full of teen-aged girls that are bishoujo. In real life, though, people may go for additional reasons, such as to enjoy large scale, professionally operated baths. Varieties include hot bath, cold bath, electric shock bath, radioactive radium bath, jacuzzi jet bath, Asian medicine infused bath, sauna, and outdoor open air bath, and mini pool to name a few. Some facilities may be limited as a premium service. Advertisements state that these baths have magical health benefits, but consumers and advertisers alike know that what's on the ads are largely bullshit. People go anyway to just feel refreshed and get a good nights sleep back at home.
I was at a men's public bath back in the day, sitting in an outdoor bath, chilling out and minding my own business when a little girl joined me, equally naked. While I sit in the hot bath, she's busy getting in and out of this one and the mini pool. What's going on here? Actually, there are strict anti-nudity laws. Public nudity is illegal, porn is forbidden to minors, and child pornography can get you locked up. All of these rules magically disappear in this particular case. Children should be protected, but as an exception, it's OK for a little girl to be naked in public view only if she is surrounded by a bathhouse full of naked men. (In some ways, this is also true in the US. For example at a swimming pool, a parent or guardian can bring minors of the same or opposite gender to a locker room because they need adult supervision.) Not to be too paranoid about security, but it was still jarring. In the end it all made sense, and I understood that everything just works in this universe, when I noticed a guy from afar with a large tattoo, arms folded, giving me the stink eye.