Your residence is based on your domicile. Your domicile is your permanent home - where you have your driver's license, where you are registered to vote, where you own your house, where the Post Office delivers your mail, and where you will return when you travel to other states or out of the country.
Most college students (especially undergraduates) normally retain their domicile at their parents' home while they are in college - even if out of state.
So, based on what you say, you would be a resident of Iowa. You would not file an Illinois return unless you made money in Illinois.
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