CA FTB likes to see a "complimentary copy" of the Federal filing which avoids them having to ask you for it later. IRS doesn't care about seeing your CA filing.
The amended CA return along with Schedule X should be mailed to
Franchise Tax Board
PO Box 942867
Sacramento, CA 94267-0001
according to https://www.ftb.ca.gov/forms/2021/2021-540-x-instructions.html
