Severance is never a requirement of any employer unless you have a signed employment agreement stating otherwise, or, it is a written policy of the company.
To increase your chances of a successful negotiation, choose a reasonable counter-offer. Think about the resources your former employers can offer and what you can offer in return. Employers usually do not want to engage in a lengthy negotiation, so presenting a reasonable offer may encourage them to accept to move on.
You do not get severance if you quit. Nobody is automatically entitled to any severance legally, ever, unless you were hired under a contract such as a 1099 employee and you have severance written into your agreement. Standard W-2 employees usually do not get severance.
It makes no difference how long you've been with a company so yes, it's legal to lay off any and everyone without severance. The exceptions: a union agreement requiring severance, a personal contract calling for a severance. This is usually only for executives and ``key'' people.
While you don't necessarily need a lawyer to negotiate severance, having one can be advantageous, especially in complex situations or if you're unsure about the legal and financial implications of your severance agreement.
Neither the California Labor Code nor the federal Fair Labor Standards Act require employers to offer severance agreements to departing employees. Instead, severance agreements are provided by employers to accomplish a specific goal.
Employers are not legally required to offer severance during layoffs, but many choose to do so to maintain goodwill and ease the transition for their former employees. If you are offered a severance agreement, remember that you don't have to sign it right away.