Citizens’ arrests are generally pretty restricted because it can be dangerous to have untrained civilians attempting to arrest people. They can easily make mistakes in the heat of the moment (especially if they’re personally involved in the crime) and even if they’re justified, a physical confrontation with a suspect can easily get out of hand. For that reason, the law usually encourages people to call the police rather than take matters into their own hands.
It’s not defined by where the sentence is served (although jail/prison is technically accurate). It’s defined by the magnitude of the punishment. Typically, a year or more (or death) is a felony. For what it’s worth, your source says this as well.
