Doesn't the bill explain all this pretty clearly? https://leginfo.legislature.ca.gov/faces/billTextClient.xhtm...
>> An operating system provider shall [...] provide an accessible interface at account setup that requires an account holder to indicate the birth date, age, or both, of the user [...]
>> “Operating system provider” means a person or entity that develops, licenses, or controls the operating system software on a computer, mobile device, or any other general purpose computing device.
Your hypothetical "embedded system" almost certainly neither has an account setup process in the first place, nor is it a general-purpose computing device, a mobile phone, or a computer.
> Reaction 3: how would this ever be enforced?
Pretty easily? They enforce it against the OS vendor for not providing such a process. They aren't enforcing the correctness of the age, nor are they claiming to.
> Someone needs to maliciously comply, in advance, on all California government systems.
...what? This is a law demanding compliance from OS vendors. Whose compliance is it even demanding in government systems for them to be malicious about it?