I'm not sure just exactly what the communist state of Illinois does for a BG check. It may be the same as a NIICS check but I'm not sure.
Remember this is the state (i.e. because of Chicago and racist/communist mayor Rham Emanuel mostly) that tried to prohibit handgun permits until they were hit by the Heller decision like DC was. They lost. I think at one point they tried to outlaw handgun ownership totally (again like DC) but the Heller lawsuit stopped that too.
Now folks there can get carry permits and I'm sure their criminal history is checked. From my understanding all firearm purchases have to be completed by the successful completion of a NIICS check also, so you get checked when you buy the firearm(s) and if you apply for a carry permit you get checked when you go through that. In this funky place (Illinois) you have to get a Firearms Owner ID card which also requires a check to buy firearms AND ammo and of course that requires a NIICS or similar check too with each individual purchase (I may be wrong on this).
No FOID card - no ammo, etc.
Left wing nut and lunatic Bloomberg was trying to establish this as a model for the entire nation and was willing to spend a fortune to do it.
Hasn't worked in Texas yet and many other places. If you have a LTC in Texas you don't have to go through another NIICS check when you buy a firearm and/or related item(s) but if you don't have the LTC you have to have a NIICS check done whenever you purchase a firearm however this doesn't apply to ammo, etc.,