Possible sources of salary info include
Job sites where people review and report on salary at their employer, even if anonymous its super likely that figuring out exactly who you are could be less than rocket science.
Credit reporting agencies.
Bullshit surveys where people answer questions like how much money you make.
Really smart educated guesses based on analyses of known good data from your own and similar employers/employees.
Big employers. As long as the data isn't directly shared with their fellow companies and only in fact with me they stand to benefit from me knowing how much they and their competitor pay so I can recommend the lowest salary an employee is likely to accept so they can minimize cost.
In short your employer doesn't know how much you make but I do and recommend based on that info. Further I can even perpetuate racism and sexism! If minorities and women are currently paid less I can let you know to offer less and still get applicants. If I train a computer to make that determination it can even decide such without me explicitly telling it to and we can blame the algorithm instead of me!
Go technology and go capitalism. High five!
This is of course entirely hypothetical but sufficiently trivial that I find it hard to imagine people not doing it.
> The bill also would prohibit an employer from seeking salary history information about an applicant for employment and would require an employer, upon reasonable request, to provide the pay scale for a position to an applicant for employment.
They're not allowed to try to find out from someone else, either.
https://leginfo.legislature.ca.gov/faces/billTextClient.xhtm...
I'm not a lawyer but that doesn't seem to be illegal or even a loophole its out of scope.
Similar to how your FICO score is not part of your credit report; it's a separate paid service that the credit unions pay for so they can charge their own customers.
While they may have that information, federal laws require them to have you self-report income in regards to credit offers.
I suspect this one has more to do with trying to engage you so that they may sell you more credit.
Credit agencies measure whether you pay your debts, nothing more.