Your employer should not be withholding California income tax if you did not live or work in California. All your income is Illinois income because you lived and worked in IL.
File a nonresident CA return and enter $0 for your CA wages. You should receive all your withholding back. You may want to mail your CA return since your CA wages will not match the W-2. Attach a note explaining that you lived and worked in IL and that your employer withheld CA tax by mistake. Include a copy of your W-2 and your IL return showing you reported all your income to IL. If you mail this, send it by certified mail with proof of delivery so you will have a record that CA received your return.
Give your employer Form DE-4 and check Box 3 “I claim exemption from withholding for 2021, and I certify I meet both of the conditions for exemption”.
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