Where can I get the 2020 California Download?

Did you use the Online version or the Desktop program?   If you used Online you can download your returns for 7 years.

I hit the wrong button.  Here’s the rest I was going to post

How to Access prior year online returns
https://ttlc.intuit.com/community/prior-year-return/help/how-do-i-access-my-prior-year-return/00/27010

If you can't get the side menus to open up to access the prior year..... You need to start entering some basic Personal Info in 2024 for the side menu to open up. Just continue a little ways into 2024. I had to go though about 12 screens.

If you used the Desktop CD/Download program then the only copy is on your computer and not saved or stored online. So you need to make and keep your own backups.

Or request a transcript from the IRS
https://www.irs.gov/individuals/get-transcript

Or get a copy of your return using form 4506
https://www.irs.gov/pub/irs-pdf/f4506.pdf