The article specifically mentions this which implies that it's stored on the car.

Berla’s software makes it impossible for vehicle owners to access their communications and call logs but does provide law enforcement with access

But it's immediately followed up with

Many car manufacturers are selling car owners’ data to advertisers as a revenue boosting tactic

Pretty much all new cars being sold today, most cars in the last 5 years, and a large percentage of cars sold in the last 10 all have some sort of cellular modem that reports back to home base with all sorts of info, then they turn around and sell it. GM has been doing this for 20+ years at this point with on star which is included in almost every car they've made.