Your understanding of how salaries work is correct. Aside from overtime (which I cover below), salaries are generally based on what your employer thinks your work is worth, and whatever they're willing to pay to keep it competitive with other employers. It's common to require full time salaried employees to work 40 hours per week, but that's not an inherent requirement of a salaried position. As for overtime:
You are (probably) entitled to overtime pay under California law.
Q. Are salaried employees entitled to overtime?
A. It depends. A salaried employee must be paid overtime unless they
meet the test for exempt status as defined by federal and state laws,
or unless they are specifically exempted from overtime by the
provisions of one of the Industrial Welfare Commission Wage Orders
regulating wages, hours and working conditions.
There are several links in that paragraph on the original site, but they don't go anywhere that explains what conditions lead to exempt status. I'll edit in that information if I find it.
Assuming that you're not exempted, the overtime requirement is as follows:
In California, the general overtime provisions are that a nonexempt employee…shall not be employed more than eight hours in any workday or more than 40 hours in any workweek unless he or she receives one and one-half times his or her regular rate of pay for all hours worked over eight hours in any workday and over 40 hours in the workweek.
It goes on to explain the process for calculating the "regular rate of pay" for salaried employees:
If you are paid a salary, the regular rate is determined as follows:
Multiply the monthly remuneration by 12 (months) to get the annual
Divide the annual salary by 52 (weeks) to get the weekly
Divide the weekly salary by the number of legal maximum
regular hours (40) to get the regular hourly rate.