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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Illinois and Iowa have tax reciprocity with regard to W-2 wages and salaries.
That means that, if you as an Iowa resident worked in Illinois, and your earnings were reported to you on a W-2, you would only have to file an Iowa tax return. Such earnings are not taxable by Illinois. They are taxable only by Iowa.
The only exception would be if your employer mistakenly withheld Illinois taxes. In that case, you'd have to file a non-resident Illinois return, showing zero Illinois income, in order to have those taxes refunded to you.