Both arrest and trial information are public. So, the records can be viewed by employers.
In the past, obtaining the arrest and trial records from California jurisdictions was a convoluted process (likely deliberately so) that involved going to the main Court house in person, waiting in two lines, and paying a steep "copy fee".
Enter the modern era.....
There are several companies that maintain mega data bases for a dazzling variety of public records. These companies charge a yearly fee ($600 or more?) for subscription. One can obtain the general arrest and Court information from them.
Small employers probably go to private detective agencies ( subscribing to one of these companies is a must for private detectives). For a fee, they will then search the data base for arrest and trial information and print it out.