You will file an Illinois resident return and may have to file a California nonresident return.
California says: "If the nonresident performs services from her or his home state, and the benefit is received in California, an independent contractor operating as a sole proprietor will have California source income under California Code of Regulations (CCR), title 18, Section 17951-4(c), when the business is unitary."
We'd have to know more about what you do for the California company.
Illinois requires residents to file state returns using the same filing status as federal. If you are filing together for federal you would file together for IL.
No tax is withheld on a Form 1099-NEC. In the future consider making quarterly estimated tax payments so you won't owe so much when you file.
Market-based sourcing for independent contractors
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