Most of the time a case will go to a branch court for the probable cause hearing. You'd have the States Attorney for victim/witness, and Defense Attorney or public defender for the criminal, with a judge deciding whether there is probable cause to proceed to trial. Run of the mill dope cases, hot cars, ect. willl usually go to the felony branch court. Cases more serious in nature, or cases where the state feels they have a bad hombre and don't want to take a chance with a finding of no probable cause will go to the Grand Jury, which is a rubber stamp almost.
Regular citizens are on the Grand Jury. In Cook County they serve for about a month, hearing numerous cases. A States Attorney questions witness/victim, no judge or defendant present. The Jury then decides if the case should proceed to trial by voting either True Bill or No Bill.
Thank you for the information.