What can a landlord do about rent not received during CV-19 lock-down. Is there anything available through the US stimulus or California?

They cannot just not pay. They have to provide proof they were unable to pay due to the pandemic. If they fail to do so then you evict the same as anyone else. 

The renter still owes it when this is over. Many places you just can't evict them during lockdowns. No you're not going to be reimbursed by the government..

To not pay rent in California, the person has to provide documentation of how covid-19 impacted their income and they are required to pay by Nov, 2020.

If the person simply didn't pay, you would handle the same as any other time the person didn't pay.