There is no way to say what the usual sentence would be. A first time offender might be convicted of a misdemeanor, or might go to state prison, depending on the facts of the crime.
Burglary is the entry of any structure, or a car, to commit a crime. As long as it isn't a residence (which can include houses, apartments, mobile homes and houseboats), it is a wobbler, meaning it can be a misdemeanor or a felony. The DA can decide how to charge it at the start of the case, and the judge can reduce it from a felony to a misdemeanor at several stages of the case, or even after somebody completes probation.
Residential burglary is a different story. It cannot be reduced to a misdemeanor, it's a strike under the Three Strikes law, and it is "prison presumptive," which means the judge has to send a person convicted of residential burglary to prison unless the court finds it is an unusual case where the interests of justice would be served. (If this is a young person and a first time offender, that might support this finding).
If "theft from car garages" means theft from garages attached to homes, this could very well qualify as residential burglary.
This person is facing the possibility of a substantial prison sentence and needs a good criminal defense attorney. If you can't afford to hire a lawyer, the court will appoint the public defender to represent you.
Without reviewing the charging document, the range on the possible sentences the defendant faces is quite large.
Here is a very, very important point from your question, though... you cannot burglarize an unlocked car. If a car was UNlocked, by law it cannot be a burglary. At worst, it would be theft of the items from inside the car - either petty theft for under $400 or grand theft if the items were over $400.
If the garages were attached, they would be considered residential burglaries, carrying up to 6 years for one count. Each vehicle burglary has a sentence range of 16 months, 2 years or 3 years. The laws of sentencing in California gets complex, but they can really start stacking up with multiple counts.
Each residential burglary is also a "strike" under California's three strikes laws, so the stakes are high. Time for a good attorney.
A trend in residential burglaries is to break into the car parked in the garage or in the driveway steal the remote for the garage and gain entry to the house through the garage. If this is what whoever was particpating in it is burglary whether the cars were locked or not. I suspect this was the goal as the remotes would be otherwise useless. More facts here would help.