Online slots are legal in California, as long as they are operated by a licensed casino. In order to be licensed, casinos must meet certain regulations set by the state.
These regulations include having a gambling license, verifying customer ID and age, and following other ethical guidelines.
The legality of online slots has been the subject of some debate in the past, but the current stance is that they are legal in California. This is based on the fact that online casinos are regulated by the state just like regular casinos, and they must follow the same ethical guidelines.
Additionally, online slots are not considered gambling devices under state law. So while players may be able to win money off of them, they are not technically playing for real money.