You will need to file a nonresident Illinois return because you did not actually work in Illinois. You will want to file an Illinois nonresident return to get the state withholding returned to you. (You report 0 IL income.)
If you do start work in Illinois in 2021 but keep your California residency, you will file an IL nonresident return and report only the income you earned in Illinois. You must file the California resident state tax return and report all income. So that you are not double taxed, California will apply a credit for the amount of tax you paid to Illinois on the income.
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