I went through this process, albeit in NJ, when my mother died intestate. I'm going to suggest you use TurboTax to prepare the final return but paper file it. Following the instructions in IRS Publication 559 ( https://www.irs.gov/pub/irs-pdf/p559.pdf ) you would put the new address on the 1040 form and sign as personal representative. Those instructions tell you to write DECEASED at the top of the return and allow you to check the Third Party Designee box and supply your contact info. If a refund is due, you would include a copy of IRS Form 1310 ( https://www.irs.gov/pub/irs-pdf/f1310.pdf ).
For the California 540 return, the process is similar and you should follow https://www.ftb.ca.gov/file/personal/filing-status/deceased-person.html for details.
In view of the lateness of the 2021 filing, do send the returns via certified mail (just that, no return postcard or insurance) so that you have a tracking number to check that it has been received and when it was received.
Hope this helps even though it isn't online filing.
