Correction, Cook County and (IIRC) one other county I can never seem to remember the name of are the most Anti-Trump places. The rest of Illinois isn’t really.
If it wasn’t for the red counties, the blue ones wouldn’t be there.
There’s some affluent areas in Cook County.
You’re probably thinking of Lake County, which includes all the rich North Shore suburbs. Those folks are mostly nominally Republican, but mushy on social issues and not big Trump fans, in my experience.