The key concept here is one of domicile. I think Section L of CA Publication 1031 explains this pretty well.
Once you establish your domicile in a new state, for tax purposes you become a resident of that state. You can have only one domicile at a time. For tax purposes, a change in domicile results in your becoming a part-year resident of each of the two states.
2020 Publication 1031 Guidelines for Determining Resident Status
Thus, if you changed your domicile from CA to NV during 2020, you would file as a part-year resident of CA.
Your state of residency for tax purposes is established by the location of your domicile.
**Answers are correct to the best of my ability but do not constitute tax or legal advice.
