California driver's license expires in May, can I get a RealID without visiting a DMV?

The only answer at this time is to wait for the DMV to reopen or the passport offices to reopen. The good news is you have a lot more time to get this

Due to circumstances resulting from the COVID-19 pandemic and the national emergency declaration, the Department of Homeland Security, as directed by President Donald J. Trump, is extending the REAL ID enforcement deadline beyond the current October 1, 2020 deadline. I have determined that states require a twelve-month delay and that the new deadline for REAL ID enforcement is October 1, 2021.

The reason you can't apply online is DHS requires the application to be made in-person

Travel industry groups have been advocating for the option to allow people to submit applications online, saying it would speed the rate of Real ID compliance and mitigate challenges at overburdened DMVs. However, some said they had hoped DHS would also have eliminated the requirement for an in-person visit, which critics say is outdated and unnecessary.

You can get a paper extension, online, here.

California has extended expiring drivers' licenses until May 31, 2020. Even if that does not apply to you, the fact is they will probably extend again if the COVID-19 situation warrants it. I suspect the DMV will have quite a backlog and may have to push it out again.

As far as air flight, they already have let you fly on a license up to 6 months past expiration. The deadline for state compliance to Real ID has been pushed for another year to October 2021.

As far as driving, the DMV lines have been crippling for over a year due to RealID, and California police are accustomed to seeing people with expired driver's licences. It's no big deal; at most they will want to hear that you do have a DMV appointment scheduled. So a week before your birthday, schedule an appointment, which will be months out. The real purpose is to buy you time with the cops. You can just go into the DMV anyway at a time of your choosing, regardless of any appointment. Don't push it too late; COVID-19 may take another world tour this fall.

You can apply for a passport at the post office. If you want something that fits in your wallet request a passport card for an additional fee. That will be sufficient for flying once REAL ID goes into effect.

Prepare your application ahead of time. At the post office they will just look it over send it on. You won't need to hang around long.

There are none. All first-time applications for a California Real ID must be made at a California DMV office.

This California DMV FAQ page contains the following text:

Can I get a REAL ID by renewing my current driver license or identification card by mail or online?

No. Since a REAL ID is a form of federal identification, you must visit a DMV office in person when applying for a REAL ID for the first time. You will need to show documentation proving your identity, California residency, and that you have an SSN (if eligible). View full list of all acceptable documentation

However, there ARE things you can do online ahead of time to apply for your REAL ID! Fill out the online application, make an appointment or check wait times at your local office, and create a checklist of everything you need to bring to DMV.

Once you have a REAL ID driver license or identification card, you may be eligible to renew by mail or online in the future.

You can't. First-time California REAL ID applications must be made in-person. You are not required to have a REAL ID driver's license to drive, but you are required to have a driver's license to drive. If you are unable to get a REAL ID driver's license before your current driver's license expires (taking into account the current extension to May 31), for whatever reason, it is still your responsibility to maintain a valid driver's license if you want to drive, which may mean you will have to renew your non-REAL-ID driver's license (which can be done by mail).