Jails are for very short sentences and short term pre-trial incarceration. This can lead to some really dumb "X% of people in jail Y" type states that sound outrageous until you realize that X is drawn from a very specific subset of the incarcerated population.
I wish it was as clear as string vs float, but it isn't. People get stuck in jails for 3-Undefined years all the time, all over the US. The only difference is "big state owned" and "many sizes incorporated area owned". Some states have laws that define this clearly, some don't. In many cases, even though it might be against the law, a prison is over-filled, so they outsource to county jails, and in others, the jail is over-filled so they outsource to another incarceration facility. It's a shit show, and I feel really bad for people that are caught in it.