You'll have set (and known to the prisoners) groups / divisions / gangs to which most of the prisoners belong to one degree or another. These could be racial, or by crime committed, etc. Some of these will cooperate with other groups, again the degree will vary.
Many of the jobs will have set groups getting on that crew and others will be excluded. A token outsider might be tolerated if the guards were the ones forcing it or if that person were a mule or messenger with a related group.
On many crews the prisoners will know the guards quite well, study them even.
Most of the guards will just do their jobs, but if its allowed some may indulge in sadistic or other hazing of the prisoners. Most however won't simply because they have no reason to... Unless their supervisor(s) demand such behavior.
There will be regular counts of prisoners to ensure no escapes or other stuff is going on. Movement will be regulated. Nobody goes anywhere unless they are supposed to be there. There might be controlled movements at specific times for specific lengths. Failure to move on time and promptly might result in punishment.
The guards will regularly search the facility for contraband. They may search prisoners entering and leaving certain areas every time.
Prisoners may view the particular crime of a prisoner as "unacceptable," say child molesting or rape and such a criminal will be set on by the other prisoners for jail house judgement if it comes to that.
Fights and assaults happen for a reason. They are not some random event and even two prisoners with differences won't do it unless it gets really serious. Neither will want retribution.
The prisoners have rules among themselves too. For example theft or stealing is a major no-no. There are likely prisoners willing to exchange "favors" for stuff so forced rape and the like is uncommon.
Talking to, sitting with, doing things with, etc., those in opposed groups, or of the wrong race, etc., will get you problems.
For things that are privileges or in short supply there will be a known schedule for use among the groups. Even within their own group one will have to wait their turn for use.
Those who break the rules get extra punishment. Solitary, a flaying, whatever the prison does in that respect. Almost anything other than doing exactly what you are supposed to be doing as a prisoner is a potential infraction.
Unless this is a death camp or the like, it will be as clean and neat as possible. There is lots of cheap labor in prisons and no shortage of prisoners to clean stuff. If you don't have particular skills the guards / staff want or need for a job you are likely to be a grounds keeper or janitor. Depending on what's available you will have a specific area to keep clean and not only the guards but the other prisoners will expect you to do that. They want it clean too. That goes double for the cell you are in.
The layout will depend on the technology available to the prison. There are likely to be several fences or walls surrounding the place. These might be some distance from the actual "yard" where things go on. That makes just getting to them difficult. If tunneling is an issue there might be solid below ground barriers to escape.
Random patrols outside the fence would be the norm. There might be fixed manned positions like towers too. Getting too close to the fence gets you warned or shot.
Inside the facility virtually every door stays locked 24 / 7 and you can only get though if a guard lets you through. No guard has sufficient keys (or whatever they use) to allow a prisoner to get very far, nor will the guards bother with you taking hostages. They might negotiate or not if you do depending on the rules.
They will have to line up for everything. Movement, food, commissary (if there is one), laundry, you-name-it. They likely will have to wear prison uniforms at all times and have no alternative clothes.
There will be a black market for stuff like stolen food, comfort items (an extra blanket or the like), and other stuff. Homemade weapons will be available more for protection from other prisoners than for something like a riot or escape.
Patrons could take the form of an prisoner who:
Needs somebody getting out soon to do something for him. Criminal or otherwise.
Needs somebody who has access to something within the prison he needs desperately.
If a character has useful skills (they're a medic, a mechanical genius, a real deal fighter, etc.) they might be in demand by prisoner leaders for their skills. A good medic is an alternative to the drunk prison doctor. A mechanical genus can make useful things. A skilled combatant makes a good body guard.
Depending on the way things are run it might be possible to smuggle in what they need to make an escape attempt. Outside help is a big plus to success. Do note, that even in a mass escape many prisoners might not participate for various reasons.
The best escape plans usually are the least complex too. Digging a tunnel takes lots of time and you have to hide the dirt, etc. The longer the plan takes, the more "stuff" it requires, and the more people involved the more likely it will be discovered.
Snitches abound. The prisoners really can't trust anyone not to snitch on them. This may be done for favorable treatment, a transfer, or other reward.
