Motor Voter Law of 1993
The National Voter Registration Act of 1993 (NVRA) (52 U.S.C. §§ 20501–20511) (formerly 42 U.S.C. §§ 1973gg–1973gg-10), also known as the Motor Voter Act, is a United States federal law signed into law by President Bill Clinton on May 20, 1993, and which came into effect on January 1, 1995. The law was enacted under the Elections Clause of the United States Constitution. The law advances voting rights in the United States by requiring state governments to offer voter registration opportunities to any eligible person who applies for or renews a driver license or applies for public assistance along with requiring the United States Postal Service to mail election materials of a state as if the state is a nonprofit.[1] The law requires states to register applicants that use a federal voter registration form to apply and prohibits states from removing registered voters from the voter rolls unless certain criteria are met.
California Drivers license aplication has a box you have to check if your not a legal resident
and depending on the county you automatically get registered as a absentee voter
https://en.wikipedia.org/wiki/National_Voter_Registration_Act_of_1993