Go to this website for information - https://www.investopedia.com/terms/d/domicile.asp

Legally, your domicile is the location you declare in legal documents, such as the address you use to vote, bank, register vehicles, and pay taxes.

I don't understand why this is a question.

Your domicile is your permanent home.  It is where you have significant financial, social, and other ties. There is no single determining factor, but things to consider are your doctor, dentist, social clubs, voter registration, and so on.  You only have one domicile at a time.

If you plan to move to Nevada in 2021, and you intend it to be a permanent move, then it will be your domicile when you move.  It is not your domicile NOW, if that's what you are asking, only after you make it your permanent home.  You move when you move, don't over-think it. 

If you want to make Nevada your domicile before you actually physically move, that will be tricky and will require substantial effort on your part to show it is not a sham.

Why does it matter?  For taxation of your pension maybe?

You cannot "claim" NV as your domicile until you establish your permanent residency there and abandon your domicile in CA.  You can only have one domicile at a time.

See "Change of Domicile" on page 10 of this reference:  https://www.ftb.ca.gov/forms/2019/2019-1031-publication.pdf