There are some interesting threads about this in the diagnosed forum. I suggest you visit this forum for a broader range and more recent descriptions.
As @Catherine says experiences vary. I think it’s often a totally different experience if you self-admit than if you go against your will. You often bring out of it what you bring in and your attitude will color the experience. It is indeed a mental hospital, because your experience there will be colored by the state of your mind.
I’ll answer your questions based on my experience from a week long visit 35 years ago. I have visited people in the hospital here and there over the years and things don’t seem to have changed much. The only difference is that at least in the US there are relatively few “mental hospitals” anymore. Now you go to a regular hospital and stay in a Psychiatric ward. Personally I think this is a good thing, because it destigmatizes things. There’s really no reason to distinguish between types of illness so much so that mental illness treatment requires a jail-like setting.
A typical day for me was comprised of breakfast in a cafeteria. I could have coffee along with standard breakfast fare. The rest of the day was comprised of group ‘activities’ which could be games, listening to music, arts and crafts, outdoor activities and so on. I played cards with a volunteer on my second day there and she was surprised that I’d only been there only one night. There was free time as well where you could socialize with fellow patients. I don’t recall any group therapy and contact with the psychiatrist was very minimal. I spoke more with a case manager/social worker than anyone else aside from the nurses and other staff.
We had day trips. I went to a mall and a local attraction with a group. I also had a pass to leave to grounds with my parents to see my psychologist.
You stay as long as is needed. At my hospital there was sort of a patient-run group meeting where they discussed people leaving the hospital. I think it was a bit of a rubber-stamp affair, but I think if you are admitted voluntarily and your insurance and the hospital was willing to give you a bed, you could stay as long or as little as you like.
There were rules. Generally, you need to follow staff requests and smoke in designated places, refrain from stealing and fighting from fellow patients and staff, there was also a no fraternization rule. People were generally punished by losing privileges and being put in isolation in time-out rooms.
I shared a dormitory-style room with another patient. The ward was separated by sex in two wings.
The staff are nice to you if you are nice to them. I was generally nice to staff and other patients and they were generally nice to me,. If you misbehaved people became more cross and stern with you, which is understandable. I never saw staff mistreat or berate patients, even though some patients often mistreated and berated them. You will be treated well if you behave well.
In similar threads on the DX forum the experience was compared to being at summer camp or on a cruise. It was very much not what I expected, and most of my fear and trepidation came from outdated misrepresentations in movies and other media.
The best representation I’ve seen lately of the experience is from a two-part House episode called “Broken 1 & 2” I say House rather than House MD, because Dr. House had his medical license suspended while he was hospitalized. While highly dramatized, the episodes do a good job of breaking stereotypes of outdated and inaccurate portrayals made in film-- especially One Flew Over the Cuckoos Nest. Lin Manuel Miranda of Hamilton and Moana fame plays a bipolar patient in it.
You don’t say what country you are in, but I’ll include a few links to videos of this which may be blocked outside of the US.