Disclaimer, I'm not a lawyer, this isn't legal advice. I've dealt with this part of the code as both an employee and an employer.
California labor law section 2870
(a) Any provision in an employment agreement which provides that an employee shall assign, or offer to assign, any of his or her rights in an invention to his or her employer shall not apply to an invention that the employee developed entirely on his or her own time without using the employer’s equipment, supplies, facilities, or trade secret information except ...[1]
It then goes on to carve out some exceptions. The one that Google leans on is this one:
(1) Relate at the time of conception or reduction to practice of the invention to the employer’s business, or actual or demonstrably anticipated research or development of the employer;
Google argues that they anticipate researching and developing anything you might be interested in working on so its theirs. In my case I pushed legal to say, in writing, that even if I wrote an iOS kitchen timer application on my own time with my own equipment, then Google would own that app. My attorney suggested that if I sued them they would likely lose, but if I sued them they would stop employing me anyway, so if I really wanted to go into the Kitchen Timer app business I should just quit rather than risk Google making a claim to my IP that I would have to pay expensive legal fees to get invalidated.
This section of the code has been litigated many times and the general consensus is time that you are both not at work, and you aren't required to be at work, can be considered "your own time" by this statute.
[1] https://leginfo.legislature.ca.gov/faces/codes_displaySectio...