If you want an "official" California digital vaccination card...

You can get one (assuming you were vaccinated in California) by going to:

http://myvaccinerecord.cdph.ca.gov/

and following the instructions. Yours truly can verify that it works if you were vaccinated at one of the LA County sites or at UCLA. You will be asked name and date of birth and to create a four-digit ID number.

It's easy to do. But, on the other hand, the end product isn't much more than you would get by taking a photo of your vaccination card and then keeping it on your smartphone.